Flexible Circuits: Something New For Everyone

As seen in the March issue of PCB007 Magazine:  http://iconnect007.uberflip.com/i/950499-pcb-mar2018/62

Just the other day, I was recording a podcast with Altium discussing flexible circuit cost drivers.  During that discussion, I was asked a question about what I see as a trend in the market.  My first thought was that I am seeing an increase in frequency of questions coming from people that are just new to flex and rigid flex design.  There are enough idiosyncrasies with flex, people are a little unsure and are reaching out with questions.  Around this same time, I had been contemplating what would be a good topic to write about for the New Technology theme of this month’s magazine.  The lightbulb went off, with flex and rigid flex, there is such a range of experience, comfort and skill that most everyone feels they are working with new technology.

Single and Double Sided Flex:

Single layer flex, flex with one layer of copper, is a place many new to flexible circuits start.  This is used when all conductors can be routed on one layer of copper.  This may be replacing wire, solving a packaging problem or even be used for aesthetic reasons in a package that will be visible to the end user.  When circuitry can’t be routed on a single layer, or shielding is needed, the progression is to move to double sided (2 copper layer) flex, or even multilayer flex.

If single and double sided flexible circuits are a new technology for you, material selection can be daunting.  There are many material options to consider, but the predominant material is rolled annealed copper/ polyimide laminate. Within this material type, there are two different options.  Adhesive based, with either acrylic adhesive or flame retardant adhesive or adhesiveless material.  Many single and double sided designs will use the adhesive based materials.  These materials are often less expensive than the adhesiveless version.  Laminates are typically provided with ½ oz. to 2 oz. copper and ½ mil  to 6 mil polyimide.  The most commonly spec’d materials tend to be ½ and 1 oz. copper with 1 mil or 2 mil polyimide and because they are the most common materials, pricing tends be lower and fabricators will often have this material in stock.  Adhesiveless materials are most often recommended for higher layer count flex designs and rigid flex construction.

Rigid Flex:

Rigid flex construction consists of a flexible section and rigid section on the same board.  What differentiates this construction from flex with a stiffener is that plated through holes extend through both the rigid and flexible section.  This construction is most often used when the design requires dense surface mount pads on both sides of the circuit.

If rigid flex is a new technology for you, there are a few key things to keep in mind.  The term “bikini cut” is important.  It is recommended to keep the adhesive within .050” of the edge of the rigid portion of the design.  Adhesiveless flex materials should be used and coverlay should not extend into the plated through areas.  There is a z-axis mis-match between the rigid materials and the adhesive that can impact the reliability of the design.

The simplest version of a rigid flex construction is to keep all plated through holes in the rigid area of the designs.  It is certainly possible to create a rigid flex with plated through holes in the flex regions as well, but this type of design requires additional processing, adding cost to the design.

The flex layers can also be “bonded” or “unbonded”.  If there are several flex layers or flexibility is a concern, one common solution is to eliminate the adhesive between selected flex layers, providing more flexibility to the overall design.  Often times this is confused with bookbinder rigid flex construction.

Bookbinder Rigid Flex

Bookbinder construction has been around for decades, but seems to be regaining popularity in the market.  A bookbinder rigid flex is similar to a hard covered book.  The flex layers are staggered, each flex layer gaining length as it is stacked on the bend so that when the flex area is bent, it does not buckle and create stress on the flex layers.  Bookbinder construction is both labor and engineering intensive and there are only a handful of fabricators that specialize in this construction.

If bookbinder rigid flex is new to you, attention should be given to the variables that need to be considered to allow the proper fit.  It is advisable to add extra length if air circulation is required to keep the flex cool in a high current application rather than tightly nest the layers.  It is also important to plan for the mechanical space this bulge will require in final assembly.  Moving along the technology scale would be dual bend bookbinder rigid flex, which includes multiple bookbinding areas that do not all bend it the same direction causing a hump on both sides of the board.

Additive Process, Sub 1 mil line and space

Using an additive process, rather than a subtractive etch process to form the circuitry, opens up significant advantages in the HDI and fine line market.  The process I am most familiar with uses a special catalytic precursor “ink” that can imaged to create the patterns or areas where conducting metal is to be deposited.  The ink controls the horizontal dimensions of the line width and spacing and the vertical dimension is controlled using an additive plating process that deposits metal only on the patterns defined by the ink.

If this additive process is new technology for you, this is your chance to use your imagination and think outside of the box.  Vias can be drilled prior to the metallization process and are then plated at the same time that the surface conductors are formed, eliminating several process steps. This process can deliver fine lines down to 5 micron in width.    There is a significant advantage to RF designs with this process.  Because the traces are formed with an additive process, the trapezoidal effect from the subtractive etch process is removed.  This process also offers the option of using metals other than copper, which is critical for applications with biocompatibility concerns.

Whether you are new to single and double sided flex, moving into rigid flex construction, thinking of using bookbinder technology, or investigating an additive process, working with new technology can be both exciting and challenging.  My best advice when working with flex and rigid flex is to involve your fabricators as early in the design process as possible.  They work with this technology every day, have an enormous wealth of knowledge and are happy to share and guide designers as they learn and adjust to new technology.


Customers: Invited Guests to the Party

Hanging on the wall in my office is a quote from Jeff Bezos that says, “We see our customers as invited guests to a party, and we are the hosts.  It’s our job every day to make every important aspect of the customer experience a little bit better.”

For this column, following the theme of “who is my customer?”, I am going to use this quote to explain my thoughts on how we define who our customers are and how we interact with our customers.  I am sure the people that know me well and have attended one of our Geek-A-Palooza events are shaking their heads and thinking, “of course she is going to talk about a party!”

Invited Guests:

Isn’t it interesting to think of our customers as invited guests? As someone in PCB sales, the quick answer to “who is my customer” is anyone that is using printed circuit boards.  From there it is traditional to break that down by industry sector, or company size, or technology.  Once that scope is narrowed down, marketing and sales craft their message to best reach those defined segments.  Sales people identify a list of target companies; find prospects at those companies and work hard to differentiate their technology or services to purchasers of printed circuit boards.

But what if “who is my customer?” was broken down using the criteria/framework of invited guest?  Wouldn’t this change the traditional model of sales person trying to win over the prospect to one of two people, or two companies, working together in a mutually beneficial relationship?  While we are ruminating over this, lets expand the definition of customer to people that can influence the customer’s decisions, expand our guest list and invite them to the party also.

When you plan an event, who do you invite?  People that you enjoy being around, people that you trust people that you respect?  Isn’t that who “anyone that uses PCB’s” would want to buy from and as equally important, isn’t that who we would want to sell to?  I cannot think of one customer that I have that I would not “invite to a party”.  I don’t know if it has been a conscious or subconscious effort, but my customers are people that I truly enjoy working with.  There is a mutual respect and trust.

Working with custom products, that often push limits of technology, there are always going to be difficulties and issues that come up, but working with people that you like, trust and respects goes a long way to resolving those issues quickly and easily.

Instead of targeting customers based on industry sector, company size or technology, what if we targeted potential customers based on how they like to do business and how that fits with how we like to do business?

There are customers that like detailed negotiations and sales people that thrive on negotiating price and terms.  There are customers that don’t spend time on those details and want to work with someone they can trust to run with the program and handle the details… and there are sales people that prefer being a trusted resource and not having to negotiate all the finer points. There are customers that view business transactionally, and suppliers that do the same.  There are customers that build long-term relationships, and suppliers that see beyond short-term issues and operate with the long-term goals in mind.

Hosting a party

The host of a party is ultimately responsible for making sure that their guests are enjoying themselves.  Isn’t that what we want for our customers too?  We want to them to have a favorable, positive experience and want to do business with us again.  Have you ever hosted a party hoping your guests would think it was average?  Of course not!

Have you ever used a football party as a reason to go get that big screen TV?  How about served your guests fancy hordevours, butler style instead of a traditional buffet?   Maybe live music at the party as a treat when people aren’t expecting it?  For all of you Geek-A-Palooza attendees, how about the ring toss as an unexpected party game?  Aren’t these added touches similar to the added value services we like to provide to our customers?  Do the added value services we provide add to the overall customer experience?  How can we do better?

In the PCB industry, how do we ensure our customers have an unexpectedly positive experience?  It is very easy to focus on product, but for this discussion, let’s take that out of the equation and assume that high quality product is delivered on time.  What are the other intangible things that are important to our customers that could make their job a little easier and provide a chance to really wow them?

The success of any event lies in the details.  Do we take enough time to dig into the details with our customers?  What are the different touchpoints that our customers have with us?   What is important to them? What is the message that we send?  Do we make it easy for our customers to share the things that are important to them?  Do we put the same level of thought and planning into our customers experience that would into hosting and planning an event?

Isn’t our marketing program similar to inviting them to the party?  All of our communication, from advertising to our website, to customer service, should be consistent and engaging, exactly like an event invitation would be.

Working in an industry that manufactures custom products, it is natural to place the focus on the product and technology.   But, we can’t forget the people and that business is built by people interacting with each other. The next time your customer places an order will they experience, “the beer is in the fridge, help yourself, I’ll be on the couch watching the game” or will they have an unexpectedly positive experience that they talk about for days to come?

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Omni PCB

PCB Advisor


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


FlexFactor: Advanced Manufacturing and Entreprenuership

Take just a minute and read through this list of new product ideas.  Can you identify the common thread?  Yes, they are all enabled by advanced technology, but would you believe that these are all products that have been pitched in the last year by high school students?

 Drive Alert:

Problem – Drowsy driving.

Solution – A patch placed on the temple can detect if the user is drowsy and wakes the user up.

Technology – Circuits form a flexible patch with sensors detecting theta brain waves indicating if the user is drowsy or daydreaming.

Fast Asleep:

Problem – In 2015, there were nearly 4,000 Sudden Unexpected Infant Deaths (SUIDS) in the U.S. with 1,600 of those being attributed to Sudden Infant Death Syndrome (SIDS).  Because of this, 59% of working parents do not get enough sleep.

Solution – A small wristband fits snugly around baby’s arm while sleeping and measures movements, oxygen and heart rate to let you know the baby is safe and sound asleep.

Technology – Mounted flexible hybrid electronic wristband connected to device via Bluetooth.

RA Solutions:

Problem – RA is a chronic, inflammatory disease that causes mild to severe joint pain and stiffness, which can lead to a wide variety of damaged body systems including skin, eyes and lungs.

Solution – The Relieve Sleeve, a pain reliever that administers heating sensations and applies electric pulses tailored to a user’s needs.  The functions are embedded in a compression sleeve for easy application around joints and muscles.

Technology – Flexible battery, micro coiling embedded in the compression fabric, Bluetooth chip to connect to app, electrical pulses produced form thin wires, wireless charging hub for battery.


Problem – According to CDC, 25.7 million Americans suffer from asthma.  Between the years 1996–2012, 8% of Olympic athletes suffered from asthma.  11.8% of the 7.8 million high school athletes in the U.S. have asthma.

Solution – A chest band with a smart patch to detect asthma symptoms and triggers and administer medication via auto-injector.

Technology – Smart fibers within the band detect symptoms of an attack specific to the individual.

Just to reiterate, these creative new product ideas have all been pitched by high school students!  These students have all participated in the NextFlex, FlexFactor program. Growing the next generation advanced manufacturing workforce is a key component of the NextFlex mission of developing technologies for commercial adoption while supporting a sustainable manufacturing ecosystem.

What is FlexFactor?

This program is designed to enable youth to engage with next generation technology through entrepreneurial immersion. Over the course of this program, students work in teams to conceptualize a Flexible Hybrid Electronics, or FHE-based hardware device that solves a human health issue or performance monitoring program, develop a business model around the opportunity, and pitch “shark tank” style to a panel of industry representatives. In the process, students become immersed in advanced technology and entrepreneurship, are inspired by the advanced manufacturing industry segment, and gain a deeper understanding of the education and career pathways for the future.

This four-week program kicks off in the classroom where students break into teams, are given the mission, the building blocks of flexible hybrid technology, and have the opportunity to define the problem and research hypothesis. Throughout the program, students interact with assigned technical mentors as they develop their product idea. The next week students take a field trip to tour an advanced manufacturing facility which provides a deep dive into the world of hybrid and flexible technology and gives the students the opportunity to interact with manufacturers, technicians and engineers to get a sense of what is like to work in these environments. The third week is a field trip to a local community college where students sit in on two 90-minute entrepreneurship lectures and get to experience the feel of college life. The final week each team pitches their product idea, including target market analysis and cost vs. revenue projections, to industry experts. Through this program, each student is enrolled in the community college and receives college credit upon completion.

How much fun would that be! With enrollment skyrocketing, the program is obviously engaging students and sparking interest. Brynt Parmeter, director of Workforce Development at NextFlex, explained that the first session started in the fall of 2016 with eight students participating. Following its fourth session the spring of 2018, the program will have had over 2,000 participants. That is amazing growth and speaks volumes about the program.

I had the opportunity to speak to Jordan Tachibana, whose was part of the group responsible for the Asthmex product listed above.  Jordan had been taking business classes with a marketing major focus and was introduced to this program through a teacher describing the program as an entrepreunuership program with an emphasis on advanced manufacturing.  “Touring the advanced manufacturing facility is actually what inspired us the most because we got to see all the cool applications and kinds of technology.  That is really what sling-shoted my idea with my group.  After seeing the technology, we saw what was possible and said, let’s go.”  Jordan is continuing to pursue the Asthmex product and has enrolled in the program for a second time.

Through FlexFactor’s collaborative approach to education, entrepreneurship and technology, NextFlex is helping students identify and engage in career pathways in advanced manufacturing, while actively increasing the interest and talent in the U.S.-based STEAM pipeline. The FlexFactor program creates a win for all stakeholders. High schools are able to expose youth to real-world problems, blend STEAM and entrepreneurship in a project learning environment, and students further develop personal and professional skills. Community colleges are able to expand enrollment, create additional educational and career pathway opportunities, and link community college STEAM focus to a Manufacturing USA Institute. Government facilitates the creation of a nationally competitive talent pool prepared to tackle society-wide technology challenges, increase student awareness in STEAM occupations, motivate students to purse STEAM education, and provide students a government-sponsored activity that develops disciplinary based knowledge and promotes critical thinking, reasoning and communication skills. Our industry benefits by the expanded awareness and interest in advanced manufacturing, reduced hiring expenses via a direct channel to qualified resources for both internship and long-term workforce requirements, and increased community exposure through relationships with local high schools and community colleges.

A talent pipeline shortage is looming across all flexible hybrid electronics (FHE) manufacturing occupations. This was validated in a 2016 study for NextFlex[1] by the Workforce Intelligence Network.  This study reported tha 25% of the workforce in FHE is over 55 years old, while only 6% is under the age of 24, indicating a talent shortage that the industry will face as experienced workers retire. There is also enthusiasm in the industry to connect with students that are unsure of career opportunities in advanced manufacturing sectors and are unaware of new technologies now in development that will impact lives in meaningful ways. The FlexFactor program is working to bridge the gap, connect students and organizations, and bring excitement about advanced manufacturing to young people.

What’s not to be excited about: mouth guards that could detect an athlete’s hydration level; non-obstructive patches that could detect blood glucose levels for Type 1 diabetics; an allergen medication patch that would administer the exact amount of epinephrine needed based on inflammation detected in the blood; and a device that can detect the levels of leptin hormone to evaluate sleep quality. Yes, these and many more cool applications are coming from the FlexFactor list of products that have been pitched. If these ideas are generated from a four-week program, I am excited to see what these students will develop in the future, and have renewed faith in the future of our industry and the role that advanced manufacturing and flexible hybrid electronics will play.


  1. Report available from nextflex.us.




PCB and EMS Process Engineering: The Man Behind the Curtain

“Pay no attention to the man behind the curtain.”  This famous quote from The Wizard of Oz, conjurs up the image of Dorothy, the Tin Man, the Cowardly Lion and the Scarecrow discovering that the great Wizard of Oz isn’t as grand or as magical as he seems.  He is in fact just a guy operating a bunch of controls behind the green curtain.  Today, references to “man behind the curtain” imply someone making decisions and making things happen behind the scenes.  Process engineering at EMS companies and PCB fabricators could be considered “the man behind the curtain”.

I was speaking with Holly Olsen from Electronics System Inc, discussing the fact that customer’s visiting and touring their PCB fabricator or EMS supplier to learn the processes and challenges encountered when building this custom engineered product is something that does not happen nearly as often as it used to.  Without these visits, knowledge of some of the behind the scenes decisions that are made day in and day out to help ensure the best yields are easy to overlook.  Holley and I thought it would be interesting to touch on a few of the key processes and decisions that are made behind the scenes and are often invisible to the customer.


The deliverable is an assembled printed circuit board.  But, throughout processing at both the EMS provider and the PCB fabricator, product is manufactured in larger panels and then broken down to the final product.

Most EMS providers prefer to specify their own panelization.  This allows them to determine fiducial size and location, tooling holes and break off points that best suit their unique process and equipment needs.   The size of the board, components that overhang the edge of the board, and board shape and thickness all play a part in the design of the ideal panel for processing.

Similarly, the deliverable to the EMS company is an array of parts for their further processing.  During the PCB fab process, the size of the manufacturing panel and the placement of parts, or arrays within the manufacturing panel is one factor that is adjusted depending on technology of the design.  A typical fabrication panel will be 18” x 24” or 21” x 24”.  As technology requirements increase, the panel size used will decrease.  Panel size is typically reduced to 12” x 18” when tight features are required or when the process requires tighter registration than standard processing is expected to meet.  With the smaller panel size, there is less impact from standard material movement.  When processing on a 12” x 18” panel does not yield the anticipated results due to material movement, process engineering may suggest using the center or “sweet spot” on the panel to minimize that impact even further.

Processing Thin Materials:

Again, this is an area that both fabrication and EMS work their magic behind the curtain.  From the EMS perspective, PCB’s that are less than .031” or flex materials require additional support for processing.  Thin materials will flex and move during SMT.  A common method to stabilize the array is to create a SMT pallet.  Pallets will cycle through and be reused in the process.  The number of pallets needed is determined by the manufacturing lot size and SMT cycle time to ensure proper manufacturing flow is maintained.

SMT pallets must be made to withstand high temperatures and cannot conduct heat allowing them to go through the reflow process.  CDM Durapol ESD is a commonly used composite material that can withstand the high temperatures and includes static dissipative characteristics.  Tension pins are designed into the pallet to align with the tooling holes on the PCB panel securing the board in place through processing.

Similarly, fabricators take special precautions with thin materials.  Often special carrier panels are used to transport the product from location to location and operators are trained to handle materials picking up from opposing corners to eliminate flex in the material.  Any dents or ding in the copper will have a high probability of creating scrap as the circuit patter is created.  Because automated equipment is not specifically designed for thin materials, leader panels are often taped to the manufacturing panel to provide additional support moving through automated equipment.

Stencil Design:

“The screen printing process is one of the most critical steps in the SMT process”, says Kevin Buffington, Manufacturing Engineer for Electronic Systems, Inc., “The combination of a good stencil and solder paste inspection is vital to the outcome of placement and reflow.”  The screen printing process begins with a well-designed SMT stencil.  Proper volume and placement of solder paste is crucial to the reduction or elimination of solder defects such as insufficient solder, shorts, and solder balls.  This is achieved by choosing the right foil thickness and aperture size for the mix of components on the printed circuit board.  Stencil design parameters are developed based on aspect ratio and area ratios of components.  These ratios are a calculation of the size of the stencil opening and the stencil thickness that allows the solder paste to release.  While not preferred, in limited cases where some very large components are included on a design with very small, fine-pitch components, step stencils may also be used to ensure that enough solder paste is deposited for the larger components.  Step stencils, as the name suggests, step up the stencil thickness in a specific area to place a greater amount of solder paste.

Framed stencils can either be fixed frame or universal frame.  Stencils will typically range from 15” x 15” to 23” x 23” and use either a solid or hollow aluminum frame.  As the name suggests the fixed stencil is permanently fixed to the frame.  The universal frame, the stencil is held within the frame which makes it possible to make changes as they are needed.

 While IPC specifies best practices for component spacing in design, today’s designers are expected to do more with less space and more often need to push the envelope of standard processing.  Fabricators and EMS providers are continually refining their processes to meet tighter pitch components and tighter pitch trace and space.   Involving your PCB fabricator and EMS provider early in the design helps to ensure that manufacturability is designed into the product.  Even better, plan a facility tour and gain a little insight into what goes on behind the curtain when manufacturing PCB’s and PCBA’s!

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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!

Updated Flex Design Guideline: Best Practices and Cost Trade-Off’s

We are very excited to announce that we have updated our flex circuit design guidelines.

This updated version focuses specifically on best practices and cost trade-off considerations to be aware of.
If you would like a copy, simply email me at: tarad@omnipcb.com  and I will be happy to forward it to you.