The Power of Flexible Circuits

I have a story to share.  Picture a beautiful, fall day filled with sunshine and warm winds.  Just after dinner a family is out in their yard.  The father and older son are finishing chores before dark and a little boy is playing in the yard with the family puppy.  In a split second that puppy ran into one a thick, dense cornfield and this four year old boy followed, chasing after his favorite pet.  It was one of those split second moments that every parent fears.   If you have ever been in a corn field in the Midwest, you know, your visibility is limited to maybe a row or two in front of you and maybe a row or two to the sides.

It quickly became apparent that this family was going to need help and law enforcement was called.  Within an hour there were 160 trained volunteers from the surrounding communities and a command center was set up.  Ten years ago, this would have been what you would typically think of as a man-hunt with chains of people walking through the fields.  The scene looked significantly different on this fall day.  The command center was able to utilize drones enabled with infrared and heat sensing technology, a helicopter with similar technology to cover a larger area, the GPS from the tractor that planted that field was able to accurately display where each and every stalk of corn was and all 160 of those searchers were able to communicate real-time.

So, why do I tell you this story in a flexible circuit column?  I tell this story, because each of those items just listed contain a flexible circuit.  Our industry accomplishes some pretty amazing things.  We regularly hear that flex and rigid flex is a significantly growing portion of the world wide PCB market but, speaking only for myself, I don’t always take time to really think about the end applications that flex enables.  Let’s look at a few of the known benefits of flexible circuits and what type of products we may interact with that have been, or are being, developed to take advantage of this.

Advantages of flexible Circuits:

Solve Product Packaging Problems:  Flex allows for a 3-axis connection.  It is able to bent and folded around corners eliminating the need for discrete pieces.  It is easy to think of products that take advantage of the space saving benefits of flexible circuits:  portable medical devices such as insulin pumps or heart rate monitors, hearing aids, smart phones and tablets, cameras.  As consumers, we are requiring our electronics to be smaller, lighter and at the same time have increased functionality.  Flexible materials allow designers to meet those demands.

Reduce Assembly Cost:  Flex eliminates hand-wiring and provides additional cost savings when purchasing costs for multiple wiring and component pieces are factored in.  Home monitoring bracelets and wearable electronics are a good example.  The product needs to be light weight and durable, wire and flexible circuits are both options.  A simple flex circuit eliminates time for assembly, purchasing costs and inspection costs by solving the problem with just one unit.

Reduces both weight and volume:  This is a big one.  Bulky wire harnesses and solder connections can be replaced with thin, light weight rigid flex.  It is not uncommon to see studies showing that this savings in weight and space can be near 60%.  Aerospace is a perfect example of an industry that benefits from reduction in weight and volume.  With aircraft, rockets, missiles, etc., weight is an expense.  Any opportunity to reduce weight and space translates to a product that is less expensive to operate.  The fun little TV screens that are being built into aircraft so we are always entertained, lighting systems in the airplane, engine controls, braking systems, are all products that have been able to take advantage of flexible materials.

Dynamic Flexing:  This is easy.  Anything with a hinge!  The one I use every day is my laptop.  Let’s not forget printers, disk drives, cameras, and robotic arms.

Thermal Management:  Flexible dielectrics offer a greater surface to volume ratio than round wire and this extra surface facilitate the heat away from the circuit.  Rigid PCB dielectrics often act as a thermal insulator inhibiting the flow of heat.  One area of significant growth in flexible circuit designs is the LED lighting market.  Automotive and aircraft applications, especially with the combined benefit of lighter weight and improved thermal management, are increasing the usage of flex.  Examples include headlamps, interior lighting, and interior electronics, just to name a few.  One of my favorite applications is LED lights in a pair of high top tennis shoes.  This application is not just your typical shoe that lights up when you walk; this high top was designed with an artistic LED lit pattern throughout the shoe.  It might not be the most high-tech application, but it is eye catching and fun.

Improved Aesthetics and Bio-Compatibility:  Appearance can impact decisions when the end user is exposed to functional elements of the product.   For example, a simple hand-held medical device being used in a doctor’s office had a wire that was visible to the patient.  Although the medical device was working perfectly, patients perception of and confidence in the procedure was not high.  This was traced back to patients not being comfortable with the perception of the wire.  That simple wire was replaced with a very simple flexible circuit, so simple, there were only two traces.  But, by making this simple change, the patient’s perception and confidence in the medical device skyrocketed.

Polyimide is also bio-compatible.  Most often, the polymide material is fully encapsulated before being inserted into the body.  New developments are exciting.  Polyimide laminate with gold, rather than copper traces are fully bio-compatible and being tested as sensors to be implanted into the human body.  This development is also aided by additive technology that allows trace size in the 5 to 10 micron range, significantly shrinking the package size as well.  There are exciting things on the horizon.

Intrinsically more reliable and reduce the opportunity for operator error:  Flexible circuits can significantly simplify the system design by reducing the number and levels of interconnection required.  Because the design is controlled by the artwork, the opportunity for human error is eliminated.   Aerospace is great example.  Spacecraft are subjected to many kinds of dynamic forces, especially during take-off.  In traditional PCB’s these vibrations contribute to failure.  Rigid flex are made to twist and flex and are a benefit in these harsh environments.  Solder joints, crimps, etc., are also at risk for failure in these conditions.  Flexible circuits can remove this concern by eliminating connections.

Yes, our industry has developed so many interesting, life enhancing and lifesaving products and for that we should all be proud to be a part of the growth in this market.  To finish the story I started earlier, this little boy emerged from the field, a little tired, very muddy and mostly angry that he still had not found his puppy.  Guess what.  The person stationed at the edge of the field that spotted him was able to notify his parents and the command center immediately with his cell phone.  Which, yes you guessed it, also contains a flexible circuit.

Contact us if you need any advice or assistance with your flexible circuit needs!

Tara Dunn, Omni PCB

 

Knowledge is Power: Reduce Cost and Shorten Lead Time

“What can I do to help drive cost from my design?”  This is a question that I am asked routinely.  That question is often followed by, “Can I get these faster?”  Both of these questions are even more predominant when talking about flexible circuits or rigid flex.  Flexible circuits are often thought of as a high-priced solution and truly, one wouldn’t design a flexible circuit without needing to utilize that technology for some reason.  That may be space, weight, packaging, flexing requirements or even aesthetics.

I think that most will agree that a quality product that is available when you need it is the primary concern when launching a new design.  But, that said, designing the most cost-effective solution to meet your needs is always going to be critical.  Today, I want to share my top 3 tips for reducing cost and shortening lead-time when working with flex.

Understand your fabricators capabilities:

In today’s fast-paced electronics world, designers and engineers rarely have time to visit a board shop for a facility tour to better understand the circuit board manufacturing process.   In a perfect world, everyone would have a chance to understand not only the basic process steps that these custom built products go through, but also understand the complexities that are involved with specialty products such as sequential lamination, microvias, flex and rigid-flex and even flex and rigid flex WITH sequential lamination and microvias.

In today’s market, there are many companies that manufacture flex and rigid flex.  There is also a significant difference in capabilities across the market.  Some manufacturers specialize in single sided and double sided flex, some in multilayer, some in rigid flex.  Within each of these specialties, there are companies that work with leading edge technology and some that do not.  All are capable of producing quality product.  But, when looking at ways to ensure you are not adding cost to your design, regularly working with your fabricator and understanding their capabilities and sweet spot in the market and then matching those capabilities with the requirements of the design can have a significant impact.

Here are a couple of examples.  First, you are working with two different designs.  One is a single sided flex with .010” line/space.  The second is a complex, 16 layer rigid flex with stacked microvias.  Your approved supplier list consists of three fabricators who offer flex:  Company A manufactures primarily single and double sided designs, Company B manufactures both flex and rigid flex, but typically works with designs that are 10 layers or less and Company C specializes in complex rigid flex.  It can get a little tricky.  It is very likely that the company that will have the best lead-time and pricing for a complex rigid flex will not have the best pricing for the simple flex.  If cost isn’t a factor, it can be easier to order both from the same fabricator, but if cost is a factor, then finding the best fit for each technology level is going to be most cost effective.

The second example has to do with understanding the capabilities matrix for each supplier.  It is important to understand for each supplier that you work with, what is considered standard, advanced and emerging technology.  Using drilled hole size as an example, certain manufacturers consider a .10” drill to be standard and increased costs are incurred at .008”.  With others there is no increase in cost until you reach .006” drill.  This in no way reflects on the quality of the product at each manufacturer, but more reflects their comfort level and their specific cost drivers at a certain level of technology.  Once you understand where those thresholds are, you can thoughtfully weigh the cost vs. benefit of moving beyond the “standard” technology.

Select common materials and materials that are in stock

There are many different types of material available for flexible circuits, and that number grows exponentially when you consider rigid flex construction.  To simplify, using the standard copper/polyimide laminates as an example, the laminate is available in two types, adhesive based and adhesiveless material.  For both types, there are a vast number of combinations of materials.  Copper is typically available in ¼ oz. to 2 oz. copper and polyimide thicknesses typically range from .5 mil to 6 mil.  Sounds great, right?  Absolutely!  But while all of these options are available, it does not mean that they are all commonly stocked at a fabricator or that they are low cost.  The best advice I can give when designing for cost and reduced lead-time is to work closely with your fabricator to develop a stack up.

In general terms, laminates with ½ or 1 ounce copper and 1 or 2 mil polyimide will be less expensive than other combinations.  BUT, cost and lead-time will boil down to the materials that your selected fabricator works with most regularly.  Please don’t spec an adhesive based laminate just because it should be less expensive.  If your fabricator manufactures with more adhesiveless materials (highly recommended for rigid flex), they may be purchasing laminate in enough volume that pricing is reduced and that savings will be passed along to you.  The same thing is true for lead-time, designing with materials that are in stock will eliminate the delays from material lead-time when the prototype is placed and lead-time is critical.

My recommendation is to work with your fabricator for a stack up and be clear about your requirements.  Let them know if materials are not critical and ask that they use commonly stocked materials.  That eliminates all assumptions and will result in the lowest cost, best lead-time scenario.

Communicate clearly in the fab notes

Typically, 75% of flex and rigid flex designs go on hold while being tooled at the fabricator.  A significant portion of those questions that need to be asked stem from unclear fab notes.  An unclear stack up is a very common issue with rigid flex.  Please make sure that you are clearly calling out which layers are flex and which are rigid.   If you have asked for the stack up prior to releasing the design, this is simple to include.  Flex and rigid flex designs can make people unsure and the basics are sometimes over-looked.

Another requirement that can be easily overlooked on the fab notes is the UL requirement.  There are many examples where after failing a burn test and investigating the cause, it is found that the UL requirements are clear in the assembly drawings, but not in the fab notes.  Your fabricator will not necessarily default to UL materials in the absence of the spec and the contract manufacturer will routinely separate the fab notes from the assembly drawings when asking for a flex quotation. Always clearly state any quality requirements in both the assembly drawings and the fab notes.

What do all of these have in common?  I believe the best way to reduce cost and lead-time is work with your fabricator throughout the design process and communicate requirements clearly.  They say experience is the best teacher and they work with new designs every day.  Take advantage of that knowledge!

A glimpse into PCB Sales

Here is a little sneak peek into the daily life of a PCB sales person.

Prospecting:

Somebody not in PCB sales might get a little laugh, but those of us in the trenches see this and and think, haha, is that me? Yes, it is! It is incredibly hard to get in touch with PCB buyers or designers and when someone answers the phone, we do a little happy dance. THEN if they don’t say “no, thanks”, we are SURE they will one day be a customer. We just have to be patient.

When a customer calls:

I know…..someone is reading this and thinking, “really? The PCB sales people I know like to take long lunches and spend their afternoon’s golfing. They don’t want to help me”. Old stereotypes are hard to overcome. But, I have been in PCB industry a very long time and have had the privilege to get to know many PCB sales people that are very good at what they do. In my opinion that is often because they truly enjoy getting to know their customers and helping to solve problems.

As I was thinking about what I wanted to say about PCB Sales in this column, I thought it would be both interesting and educational to ask both customers and manufacturers their thoughts on PCB Sales. I was pleasantly surprised at the enthusiastic response I received.

Question #1: In your opinion, what traits do good PCB sales people have in common?

From PCB Users:

* A better than average knowledge of PCB construction

* The ability to offer suggestions and solutions when we struggle with a new design and technology need

* Respond quickly when there is a request or issue

* Provide follow up to the details so I don’t have to worry about what is being completed

* Know the line between persistence and annoyance. PCB’s aren’t the only thing on my plate

* Excellent communication skills

* Understands when I call with an issue and helps work with manufacturing or engineering to resolve the issue so I can focus on other things

* Takes the time to learn how we prefer to work and customizes responses to fit as best as possible

* In depth knowledge of the PCB market, new materials, supply issues, etc. and provides information on what might be important to us

Got it: Knowledgeable about PCB’s and the industry, organized, strong communication skills and customer focused.

From PCB Manufacturers:

* Persistence and tenacity to follow thru and listen more than they talk

* In-depth understanding of the customer, how they like to work and what additional business is available

* Respond quickly and thoroughly

* Consistently find new opportunities and new business

* Great follow up, know their customers, aggressive when they need to be and very personable. Did I mention organized?

Got it: Knowledgeable about customer’s needs, organized, strong communication skills and brings in new business. These two lists are actually pretty similar.

Question #2: What do you wish PCB sales people did that they currently don’t, or currently don’t do well?

From PCB Users:

* Advocate for annual cost savings on behalf of the customer. This would foster trust and repeat business

* Understand our systems and market pressures outside of ordering the PCB. There are a lot of different considerations and decisions made that may not be apparent to the PCB manufacturer but are critical to us.

* Proactivity. Offer suggestions for cost or lead-time reductions. We are not the experts in PCB design and would be interested in how we can improve

* Provide the very best price the first time, especially with larger programs. Don’t come back with reduced pricing after I give you feedback. That wastes time and resources for both of us.

Got it: Detailed knowledge of customer’s business and proactively advocate for your customer’s best interest.

From PCB Manufacturer’s:

* Ask for the PO and know how to sell value, not just on price!

* Stop relying on price to differentiate and win the order

* Close more business in a timely manner

* Identify customers that find value in the quality, customer services, and fast response that we offer rather than sell on price.

Got it: Differentiate the manufacturer’s offering so the comparable factor between offers is not price alone. Interesting, this is similar to the message above also; advocate for value of the manufacturer’s strengths with your customers.

Summarizing the feedback from both customers and manufacturer’s, the most successful sales PCB sales people are organized, take a genuine interest in their customer’s needs and business challenges, have a better than average understanding of the PCB industry, fully understand the manufacturer’s strengths and capabilities and have the ability to advocate for both to find the best solution. There is room for improvement by being more proactive in solving your customer’s challenges and in understanding the differentiating value of the manufacturer to sell on total value rather than price.

You will get all you want in life if you help enough other people get what they want. – Zig Ziglar

My closing thought is that it truly is difficult to reach a comprehensive level of understanding of both the customer’s needs and the needs of PCB manufacturing. The information that is easily obtained, is often just skimming the surface of the full picture. Sales people continuously search out opportunities to interact with their customers outside of the conference room. Those relaxed conversations often offer the best glimpses into what people really need from their sales person. There is no roadmap to use, every customer has different needs. If you attend an IPC show, SMTA expo, IPC Designers Council meetings or even Geek-A-Palooza, there is no shortage of sales people trying to increase their technical knowledge and get to know others. I strongly encourage PCB users and PCB manufacturers to do the same. The more we all know about each other’s needs, the stronger the relationships will be for everyone.

Troubleshooting Flex Applications

I imagine that everyone has been in this position at one time or another, despite everyone’s best attempt at creating the perfect design, PCB fabrication and assembly, something goes wrong and the trouble shooting begins.  I had the opportunity to sit down with Ed Knutson, the President/Founder of Dimation, to swap some of our best war stories.  Ed specializes in quick turn assembly and design and I bring the fabrication piece to the discussion.  Our banter back and forth was primarily focused on flexible circuit applications for aircraft and Mil/Aero projects.  I am not sure if that is because of the more stringent requirements for those applications or more likely because that is an industry segment that we both work with regularly.   At the end of our discussion, we concluded that most of the war stories could be traced back to a break-down in communication and often times simply not fully understanding how each piece of the design-fabrication- assembly puzzle fit together.  We want to share a few of our stories and lessons learned.

UL Materials

Aircraft applications typically require materials rated to UL94V-0.  The assembly is complete and the burn test fails.  What happened?  The perfect storm.  When the design files were created for the PCB fabrication and assembly, the UL requirements were noted in the assembly files only and called out by test requirements, not UL 94V-0.  This was an ITAR application, so the PCB fab files were separated from the assembly files and forwarded the flex manufacturer.  Because there were no UL requirements listed on the fabrication notes, the supplier defaulted to their standard materials and the flex was not built with flame retardant materials.  That explains why the final assembly failed the burn test.  Lesson Learned:  Always clearly communicate UL requirements and include the requirement in both the PCB fabrication notes and the assembly notes.

Coverlay

There were many stories along this line, but this one is classic, we have both seen this more than once.  A particular application, on a tiny flex circuit, requires a very tight pad pattern. Standard, adhesive based coverlay, was called out in the stack up.  As the flex manufacturer was setting up the tooling, they asked if that area could be “gang opened” because the tight features would cause fabrication issues when aligning the drilled coverlay.   That is a very common question that I have seen asked and approved hundreds of times.  The designer agreed that would be fine and that pad location was left free of coverlay.  But, once the parts arrived at the assembler and they went to screen print the paste, the area shorted out.  The problem was ultimately solved by using photimageble coverlay to accommodate the tight feature pattern.  Lesson Learned:  Review even the “standard” requests with a critical eye for the next processing steps the flex will see after fabrication.   The size of this particular flex combined with the tight features was the perfect combination to cause an issue with something that is routinely done.

Bend radius

By definition, flexible circuits are designed to bend, fold, and flex during installation and/or use.  That doesn’t mean that the copper will not crack or break when it is overly stressed.  There are two very important things to be aware of.  First, RA (rolled annealed) copper versus ED (electodeposited) copper.  There really is a significant difference in ductility.  With a tight bend radius, or for a dynamically flexing application, specify RA copper.  Second, involve your fabricator.  The flex manufacturer is only going to see the design in a two dimensional view.  They will not know exactly how this is going to be used in your final assembly.  If you are concerned about bend radius or otherwise stressing the copper, ask for their advice.  There are many different “tricks of the trade” that a flex fabricator can recommend to ease the stress on the copper and improve performance.  Use their knowledge!

Array configuration for assembly

It is common knowledge that assembling flex can create challenges.  A lot of trial and error is done to find the best way to handle it.  Flex circuit size, array configuration, component placement and stiffener requirements all play into the decision, which just may be equal parts art and science.  The first decision is whether the assembly will be done by hand or machine.  If the assembly is not done by hand, whether to use a stiffened array or machined pallet needs to be determined.  Here are a few examples:

For a small flex, with a few components on just one side and no stiffener requirements, consider creating a FR4 stiffener pallet with adhesive on the outside perimeter only.  After assembly, the flex can easily be peeled away from the stiffener pallet.  Caution:  a stiffener pallet with adhesive in selected areas only can easily be misunderstood during fabrication.  Make the objective very clear in the fabrication files.

For a long flex with stiffeners, we suggest cross hatching the copper, or adding in a copper pattern to maintain as much of the copper in the array as possible to add stability.  The flex can be pre-routed with tabs left to hold this into the array during assembly.  Once parts have been assembled, simply cut the tabs to release the flex from the array.  Caution:  stencil tolerance over this long length is an issue to be aware of.

A custom pallet is another common choice for assembly, especially when you are running more than a few panels.  Most often this is designed with FR4 material. The benefit to this is stability and flatness during assembly and also the ability to nest the flexible circuits in the tightest configuration possible to reduce the cost of the fabrication.  There is no need to add in extra copper area in the array for stability.

These are just a few of the lessons learned that we have accumulated over time.  I hope that these provide insight and suggestion that will help with your future flex designs, or at the very least, let you commiserate and know that you are not the only one challenged with these types of issues.  Feel free to get in touch and share your stories with us!

 

Tips for Time Critical PCB’s

Can you relate to this common scenario? A quotation is received for the fabrication of three different PCB part numbers and a purchase order is placed for delivery in five days, on a time-critical project.

A few hours later, the dreaded email is received. There are questions regarding the design that are putting the project on hold. It takes a day, or possibly two, to coordinate the resolution of the questions between your customer, the PCB designer and the fabricator.

Next, you are informed that the delivery date for the PCB’s is pushed out for the two day delay in answering questions. Ugh! Now the schedule has to be adjusted, the components you paid a premium for will be sitting there waiting for the boards, and your customer is NOT happy.

This scenario occurs time and time again. Approximately 90% of designs that go through CAD/CAM at a PCB fabricator have questions that must be answered before the fabricator can start the board manufacturing. Some questions are minor and can be answered quickly; others can require a partial or complete redesign of the PCB.

Elizabeth Foradori and I sat down to discuss our thoughts and ideas on how to best work with PCB fabricators to reduce the likelihood of any delays during time-critical development of a new product.   Chapters could be written on this topic, but our hope is that these ideas provide a basis to encourage discussion early in the design process.

Prior to placing a purchase order:

Research and select your printed circuit board fabricator early in the process: If the design is going to be a standard design, on common material and fit neatly into any manufacturer’s “standard capabilities”, that makes things much easier. But, if the new design is going to be pushing the limits of standard technology in any way – microvias, fine line, tight pitch or tight tolerance, selective surface finish, exotic materials, rigid-flex – selecting a supplier early in the process, whose capabilities match the technology needed, will ensure that the design can be manufactured quickly once you are ready to release the files.

Involve the fabricator early in the design process: Ask questions. Talk to your supplier frequently during the design of the PCB. They encourage questions and are happy to make recommendations. Once the fabricator understands what you are trying to accomplish, they can make recommendations that will ensure that the design is manufacturable.   As a final step, or even an intermediate step during the design process, ask your fabricator to run a design rule check based on your files. This may not catch every issue and eliminate all engineering questions at the CAD/CAM stage, but it will catch the major issues that would require lengthy redesign once a project is released.

Verify that material is available and will be in stock when the design is complete:   Fabricators do try to stock the common materials and even small quantities of the less common materials to avoid delays. Unfortunately, they cannot stock all materials. Once the stack-up is finalized, ask the fabricator if this is material that will be in stock. If not, work with your supplier to pre-order the material to have in-house when you are ready to release the design. Some fabricators will secure material based on a simple email authorization; others will require a purchase order. Either way, planning for material to be in stock when the design is complete can save anywhere from five days to six weeks.

Once a purchase order is placed:

Send complete files: Review the files being submitted with the purchase order to ensure they are complete. Is the net list included? Are the fab notes complete, confirming any quality requirements, material specifications, and surface finish requirements? Do the fab notes match the gerber data?   These are all very common reasons that files are placed on engineering hold.

When you receive questions from the CAD/CAM tooling group, ask if this includes all questions associated with the design. Sometimes two different engineers may be working on the same design to meet an expedited delivery and both may have questions in their portion of the process. Other times, when the initial issues are encountered, the job is set aside only to find additional issues when work is resumed. The process can be streamlined by taking all questions to your designer or your end customer at one time.

If questions are fairly involved, it is always best to try to schedule a conference call between your fabricator, your designer or end customer and yourself to resolve the issue as quickly as possible. Email offers a great documentation trail for any changes, but can drag the process out longer than necessary. If communicating via conference call, ensure that someone is responsible for documenting the discussion and sending that to all parties involved.

Once the questions are answered, follow up with your supplier to confirm that the questions involved in the tooling process have not impacted your delivery schedule. Delays of a few hours are usually absorbed into the initial lead-time. Longer delays can impact delivery. PCB fabricators are typically very good about notifying customers of any changes in delivery date due to engineering questions, but it is always a good practice to ask. You don’t want to be surprised on the day you are expecting your printed circuit boards.

In summary, communication with your supplier is the best way to reduce the cycle time needed for fabrication of time-critical, new printed circuit board designs. Ask for recommendations during the design phase to ensure the design is manufacturable, verify that material will be available when the design is released, and if there are engineering questions, and communicate quickly to have those resolved.   Take advantage of the fabricators expertise and ask questions!

Contact us for further information!  www.omnipcb.com

Transition from Domestic Prototype to Off Shore Production for Flex Circuits

A smooth domestic to off shore transition

Designing a flex to be prototyped domestically? No, problem. Designing a rigid flex for production off shore? Got it. Designing a part that will be prototyped domestically with a seamless transition to off shore production? That can be a little more challenging.  We have probably all been there. The prototypes are needed on a very tight delivery schedule and are built domestically. The testing is complete and the same files are sent to an off shore manufacturer for the production build. The order is placed and suddenly, the engineering questions start coming in. Can the materials be changed? Can the hole size or pad size be altered to improve manufacturability? These common questions now require the time and effort to evaluate and ultimately the time and effort to complete the rev spin before production product can be released. We sat down with Ashley Luxton of Graphic, PLC to learn his recommendations to minimize these disruptions . Our discussion focused on the importance of supplier selection, items that are universal and key areas that have more significant variation. A link to our discussion is included here.

Supplier Selection – Choose your supplier carefully and consider the different options available. There are manufacturers that own both domestic and off shore facilities, there are domestic manufacturers that partner with off shore facilities and there are manufacturers that work only domestically or only off shore.

When working with a manufacturer that has both domestic and off shore capabilities, it is critical to communicate with them early in the design process. The fabricator, understanding both the domestic and off shore preferences and capabilities, will be happy to make recommendations for material selection, panel utilization, and also how to maximize yields for the production volumes.

A domestic supplier that partners with an off shore manufacturer will be able to offer this same type of guidance. Due diligence is recommended. Most domestic manufacturers that partner with an off shore supplier do so to offer their customers a full service option. Significant effort is put into learning their partner’s technical capabilities, material preferences and operations. The lines of communication between the facilities are well established.

There are also domestic suppliers that purchase product from off shore suppliers to support a full range of volume requirements for their customers but have not put the extra effort into learning and understanding the details of their off shore partners technical capabilities. This model provides the customer with volume production from off shore, but may not be the best solution when looking for design guidance to ensure a smooth domestic to off shore transition.

When working with two independent facilities, take the time to fully understand the off-shore suppliers capabilities and material preferences and then apply that criteria to the domestic prototype design.

Universal Criteria: Whether your PCB’s are being manufactured domestically or off shore, certain things are universal. Quality specifications such as IPC Class, FAIR requirements, and testing requirements do not change. Some of these specifications may not be as critical at the prototype stage and could be waived, but the interpretation of the specification will be consistent.

Designing to maximize yields may not be as critical with a prototype order, but with the higher volumes typically associated with off shore production, expected yields should be considered. There are universal criteria for maximized yields. Increasing holes sizes, pad sizes, line width and space will all improve yields at the manufacturer and have a direct impact on cost.

Acceptance of X-outs should also be considered. Allowing X-outs in your delivered array will have a direct impact on cost. If X-outs are not allowed, both domestic and off shore manufacturers will factor in the yield loss associated with scrapping any good pieces in an array that has an x-out. If X-outs are not allowed, this should be clearly communicated to avoid any misunderstanding.

Significant Variation: Preferred materials can vary significantly between domestic and off shore manufacturing. This preference is typically a function of material availability and cost.   Logically, off shore suppliers will prefer to use materials that are produced locally. These materials are more readily available, with lower transportation costs. Most off shore suppliers will also use the materials that are more common in the US, but pricing will be higher and lead-time longer.

Be careful not to over specify materials. Referencing the appropriate IPC slash sheet, rather than the specific material, allows more flexibility for the supplier.   This flexibility will result in lower cost and shorter lead-time. If more control is required for material selection, using an “approved list” of materials that has been tested and approved is another option that allows the manufacturer flexibility to use their more preferred materials, while giving the designer more control of materials being used.

Another aspect that varies significantly is panel utilization. Domestically, the most common panel size is 18” x 24” with 16” x 22” of useable space for the manufacturer and it is most cost effective to design the part or the array to best fit that space.   Off shore manufacturers have much more flexibility with their panel sizes, use many different panel sizes to best utilize material and generally work with larger panels. Off shore it is more critical to design the array to best utilize the material within the array and overall array size has much less of a cost impact.

To recap, when looking for the smoothest transition from domestic prototype to off shore production manufacturing, research suppliers and select a supplier that can demonstrate their knowledge of the off shore facility’s technical capabilities, material preferences and clearly have a streamlined form of communication. Quality and testing specs are universal and should transfer from one facility to the other with no issue but special attention should be given to controlled impedance, materials and panel utilization, as these can vary significantly between domestic and off shore manufacturing facilities. A smooth transition from domestic prototypes to off shore production does not need to be difficult, but it does need to be well planned.

Please contact us for more information!  www.omnipcb.com

Flex Circuit Cost Drivers

Primary Cost Drivers for Flex Circuit Designs

Someone once told me that the potential applications for flexible circuits are really only limited by our imaginations. After pondering that a bit, I had to agree. In fact, one of the things I like best about what I do is that moment during a discussion when I can see the lightbulb go off in a designer’s head. Something in our discussion, or a sample that we were looking at, triggered an idea. Flexible circuits continue to be a growing part of the printed circuit board industry. While most people are comfortable with the cost drivers of rigid PCB designs, many are not as comfortable with flex.   Although the three primary cost drivers are the same – panel utilization, materials and technology – there are subtleties of each to be mindful of with flexible circuit design. Elizabeth Foradori and I sat down to discuss these cost drivers and trade-offs. A link to that discussion is included at the end of this column.

Panel Utilization: Typically, panel utilization, or the “number up”, is the biggest cost driver for flexible circuit designs. Fabricators charge for material by the panel, so the piece part price is the panel price divided by the number of parts on the panel. As with rigid PCB designs, it is critical to understand the panel sizes that the fabricator is working with. Panel sizes are most often 12” x 18” or 18” x 24”. Fabricators commonly use the outside one-inch border of the manufacturing panel for coupons and tooling holes. Effectively, when designing, optimizing the useable space of 16” x 22” and 10” x 16” either with individual pieces or arrays, will result in the lowest cost option.

There are a few unique things about flexible circuit panelization. Flex circuits are often unusually shaped, not the standard square or rectangular shape typically seen in rigid PCBs. Standard panelization programs do not necessarily take this into account. In the example shown here, the flexible circuit is “L” shaped. Standard panelization would put six pieces per panel. But, by reverse nesting the parts, this can be increased to eight. Another thing to keep in mind is that flexible circuits are intended to be folded, bent, or flexed in use. This design could be straightened for fabrication, allowing even more efficient panel utilization with 10 pieces per panel. The “L” shape could be created once the circuit is complete. The lesson here is to not rely on the standard panelization programs, but to analyze each design with material utilization in mind.

Materials: There are many different material options for flexible circuits and the number of options is even greater when looking at rigid-flex designs. For the purposes of this discussion, the focus will be on the commonly used copper/polyimide combinations. In general, there are three types of materials: copper with acrylic adhesive and polyimide; copper with flame retardant adhesive and polyimide; and adhesiveless copper with polyimide. These materials are available in many different options ranging from ¼-ounce copper to 2-oz. copper, and 0.5-mil polyimide to 6-mil polyimide.

Assuming there is no electrical or performance reason driving material selection, choosing the materials most commonly used and stocked at the fabricator will prevent adding unnecessary cost to the design. In terms of construction, the copper-acrylic adhesive – polyimide material is most common with lower layer count designs. The flame retardant adhesive option is sometimes lower cost, but outside of a UL requirement, it is not as popular and not as commonly stocked. Adhesiveless material is more expensive, but when working with higher layer count designs and rigid-flex, this would be the material of choice based on the lower CTE value of the material.

In terms of copper and polyimide thickness, 1-oz. copper with 1- or 2-mil polyimide is most common, followed by ½-oz. copper. Material price increases quickly when going below 1-mil polyimide or increasing to 3-mil and 5-mil polyimide. This pricing also increases substantially when you move to ¼-oz. or 2-oz. copper.

Another material choice to make is using polyimide coverlay or flexible liquid photoimageable coverlay. The flexible LPI is going to be less expensive and requires less processing by the fabricator, but there are trade-offs to bear in mind when looking at dynamic flex applications and reliability. The polyimide coverlay is considered the most reliable in high flex applications.

Flexible circuit stiffeners are another material to consider. Typically, stiffeners are either FR-4 or polyimide. Flexible circuits are often rigidized with a piece of FR-4 material to help support component weight, while polyimide stiffeners may be added to increase thickness in specific areas, create a bend area, or provide a barrier in a high wear area. Both types of stiffeners can be bonded with either a pressure-sensitive adhesive (PSA) or a thermal-set adhesive. The cost driver behind each adhesive option is different, dependent on the stiffener material. If the application environment allows it, pressure-sensitive adhesive will be less expensive than thermal-set adhesive for FR-4 stiffeners. This is driven by the need for the fabricator to put the panels in an additional press cycle to cure the thermal-set adhesive. Conversely, the polyimide stiffeners are commonly placed and bonded while the circuit is still in panel form and during the same press cycle that cures the polyimide coverlay. Using PSA for the polyimide stiffeners will increase cost, due to the added labor needed to hand place these after processing through the press.

Technology: Moving on to cost drivers based on technology, line width and space and hole size are the common cost drivers in standard designs. Of course, with any type of PCB manufacturing, the bigger the better in terms of ease of manufacturability. Reaching out to several flex circuit fabricators, the most common threshold that moves from a standard process to a more advanced process is 0.004” line/space and 0.010” hole size. Anything below these will increase costs.

Multiple surface finishes and selective plating requirements also drive costs. This should be avoided if at all possible. Running the flex panels through two surface finishes is an obvious cost adder, but the cost increase is compounded by the taping and de-taping process required and the subsequent yield loss associated with that process.

Button plating is another cost adder to consider. This process creates the plated-through-hole connection without adding extra copper to the rest of the circuit. While this does increase cost, certain applications require a level of flexibility that cannot be achieved with the addition of electrodeposited copper during the manufacturing process.

Recap: To summarize, the biggest cost driver in flexible circuit design is material utilization. Take time to investigate how the flex will fit on the production panel to ensure the best use of that space. Consider material selection and if at all possible, select materials that are commonly stocked; these are also typically the lower cost materials. Before adding additional layers, use smaller line width and spacing. Stiffeners, button plating, and controlled impedance would all be considered medium cost factors, while layer count, dual surface finish requirements, and line width and space below .004” would be considered higher cost adders.

Involving the fabricator early in the design process can help avoid unnecessarily adding costs to your design. They see hundreds of flex designs each year – tap into that pool of knowledge!

Recording Link: https://youtu.be/M-t9xkbTBM0

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Price Considerations for Flexible Circuit Design

For the most part, designers understand what types of constructions and design attributes are “non-standard” and add cost to their design.  Flex circuit and rigid flex design is similar with a few “odd ball” factors thrown in.  We have compiled a short list of design attributes and how they impact cost.  For more detailed information on costs or for a free copy of our design guide, please contact us!

 Low Cost Factors:

• Complex routing and scoring
• Edge routing
• Strain relief
• LF vs AP materials

 Medium Cost Factors:

• Aspect ratio > 10:1
• Drill hole count > 30K
• Non-FR4 material in the rigid areas
• Drilled holes < .012”
• Stiffeners (rigidizers)
• Added tear stops
• Line width and space < .005”
• Button plating
• Controlled impedance
• Annular ring (pad < drill + 12)

 High Cost Factors:

• Advanced technologies
• Buried vias
• Layer count
• Material utilization
• Selective plating
• Buried access (ZIF connectors)
• Line width and space  < .004”

 Cost Trade Offs:  general rule of thumb 

• Use smaller line width/space before adding layers
• Investigate how the boards will fit in the production panel for material untilization
  • do not forget about nesting flex or straightening a bend and folding after parts are removed from the panel
  • LF materials are lower cost than AP materials, but the increase in acrylic resin in a via stack can reduce reliability due to an excessive CTE-Z

 Involve your PCB supply chain partner early in the design phase to ensure the most reliable, cost effective design that will meet your specific requirements!

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