My Thoughts on the IPC Flex and HDI Forum

As an attendee at the IPC Flexible Circuits – HDI Conference on October 28th – 30th, 2015, I found myself in a room of people, all eager for technical information, with the opportunity to reconnect with industry friends and to make new connections. The audience was diverse with young people, new to our industry, sitting alongside industry veterans willingly sharing their knowledge and passion for HDI design and flexible circuit technology.   The conference kicked off with intermediate level, half day tutorials on both flexible circuit design and HDI. The second and third days provided advanced level speaker presentations in short 45 minute segments allowing time to digest the information, speak further with the presenters and network with industry peers.

Two comments made early in the technical conference solidified an event message in my mind. Mike Carano, with RBP Chemical Technology and the Event Chair, commented in his opening remarks that networking is one the greatest opportunities with IPC. Brad Bourne with FTG gave the keynote presentation: Organizational Commitment to High Reliability. He presented the message that PCB reliability goes way beyond the manufacturing of the printed circuit board. Everyone in the industry impacts reliability: circuit board designers, raw material suppliers, fabricators, contract manufacturers and end users of the product.

Following the opening keynote, we were treated to a presentation by Andrew Schimmoeller and Jeffrey Friend with Battelle. Their riveting presentation explained the use of flexible circuits in the design of a neurological stimulation system that re-animated a paralyzed hand, controlled by patient thoughts. I have re-watched the short video on this topic several times since the event. This is an inspiring example of the things our industry can accomplish by working together.

Throughout the conference there were several presentations addressing flex and rigid flex including: design rules for flex performance, high reliability flex and rigid flex, flex vs flexibility and new developments in HDI technology. Beyond fabrication and design related information, we also learned of unique challenges with flex and rigid flex in terms of stack up and impedance, reducing fabrication challenges with new materials, metallization for HDI and flex circuit technology. The amount of relevant technical knowledge disseminated in a short time period was staggering. I was continually reminded of the power of the combined knowledge of our speakers.

While many of the presentations detailed proven technology, reliability data and design criteria we were also informed of exciting emerging technologies including the latest developments in thin film metallization and via filling and a novel new approach for applying metallization to ultra-thin substrates.  It will be interesting to follow these new technologies and see how they develop over the next few years.

The presentations on break-through technologies for high reliability and technical organizational innovations were both intriguing and thought provoking. It is always inspiring to listen to examples of people being able to “step outside of the box” and apply existing technology in new and creative ways.

In addition to the remarkable level of technical knowledge presented, there were numerous opportunities each day for networking. I personally had the privilege of meeting several new people and was able spend time with others that I haven’t seen in a while.

Attendees consistently remarked on the value gained from attending this year’s event. Todd MacFadden, Component Reliability Engineer at Bose Corporation commented, “ I am thrilled by the information and insights I gained from this forum on flex and HDI. The event was well-organized and the content was highly relevant to me. The timing of the event couldn’t have been better for me. I came with only the basic understanding of flex, just as we are starting to ramp up our need for this technology, and I am coming away with many tools and a long list of contacts to help my teams design for success. I am grateful to the IPC for assembling such a broad and diverse expertise into a concise and useful conference, and for providing plenty of opportunities to network. Great Job!”

Keith Holman, Sr. Buyer – Electronics, with Orbital ATK – Defense Systems Group remarked, “The IPC Forum was a great educational experience as well as an exceptional networking opportunity. The level of discussion around the current issues and successes with HDI and Flex/ Rigid Flex coupled with the new technology, made for great discussions. Each of the presenters was very knowledgeable about their topics and also made themselves available for further conversations. I would recommend the IPC forum to any technical or non-technical people involved with IPC.”

Ernie Kreiner, PWB Designer III, C.I.D.+ with L-3 Fusing and Ordnance Systems commented, “Attending the IPC conferences has always been an excellent experience. Networking with peers and seeing what technologies are out there and what is on the horizon, is always informative.”

I couldn’t agree with these three gentlemen more. I personally learned something from each speaker. The electronics industry is changing at a rapid pace and both flex and HDI designs are a fast growing segment of the industry.    As valuable as the technical information is all on its own, the networking component gave the time to chat with OEM’s and flex users to learn their challenges and also talk with flex fabricators and materials suppliers to learn of new programs and technologies. The conference gave me the opportunity to meet new people and expand my industry resources.   In this complex industry, there is so much value in the ability to reach out to others in different areas of the industry to help solve new challenges.

Anne Marie Mulvihill and IPC put together a cohesive and well run technical conference. Thank you to IPC for their work and dedication to educating the industry while at the same time providing networking opportunities to tie us all together.

The Battelle NeuroLife Project

A couple of weeks ago, at the IPC Flexible Circuit – HDI Forum, we had the opportunity to listen to a riveting presentation from Battelle that we felt compelled to share.
The presentation began with the video of a Tedx Talk of Chad Bouton presenting research and the subsequent success of a project designed to reconnect a paralyzed man’s brain to his body through advanced technology.

The presentation concluded with the introduction of, and words from, Ian, a paralyzed man instrumental in both the research and medical trials.

The video is just over 15 minutes long and you could have heard a pin drop.  Not even a piece of paper was shuffled,  truly amazing, when you picture a room of nearly 100 people and the distractions that we all have with our phones and email when we are out of the office.
We encourage you to watch: NeuroLife Project    
It is well worth the time!
This technology shown in this video was enabled by the use of flexible circuits. In fact, by the use of flexible circuits that pushed the limits of most standard capabilities.

Flex was chosen for several reasons: ease of manipulation on a patient, repeatable applications (stable dimensions, silkscreen marked for location identification/mapping), durability of materials, high dielectric strength, and reliable system functionality.

Not only were we fascinated by the development of this technology, we are excited to be part of an industry that pushes established boundaries of manufacturability to create products that enable these type of life changing developments.
I hope you enjoy the video as much as we did!


Click here to watch the video

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!

PCB Final Surface Finish Selection: No one size fits all solution

Remember the good ole days when hot air solder level was the go-to surface finish for almost all applications? The decision about surface finish was an easy one. The primary function of the surface finish was to protect the copper from oxidation prior to assembly. Wow, have things changed! Today’s expectations include: superior solderability; contact performance; wire bondability; corrosion and thermal resistance; extended end-use life; and of course, all at a low cost.

Common surface finishes now include HASL, both leaded and lead-free, OSP, immersion tin, immersion silver, ENIG and ENEPIG. Unfortunately, there is no one-size-fits-all surface finish that fulfills all the requirements in the industry; the decision really depends on your specific application and design.

Recently, Elizabeth Foradori and I sat down with Robyn Hanson from MacDermid Electronic Solutions to learn about the key considerations for final surface finish choice and the cautions of each from the OEM or assembly perspective. To listen to the discussion, click here. For a concise list of the pros and cons of each finish, click here. Following are some of the highlights.

Considerations for Surface Finish Choice: Does the application require lead or lead-free assembly? Will the end environment have extreme temperatures or humidity concerns? What shelf life is needed, and will it be months or years? Does the design have fine-pitch components? Is this an RF or high-frequency application? Will probe-ability be required for testing? Is thermal resistance or shock and drop resistance required?

Once these questions are answered, the surface finish options can be reviewed to find the best fit.

HASL—Hot Air Solder Leveling:

  • The oldest surface finish
  • Lead and Lead-free versions are available
  • Leaded HASL currently in limited use due to ROHS and WEEE initiatives
  • Currently exempt: industrial vehicles, military, aerospace and defense, high performance electronics
  • Leaded versions are harder to source
  • Long shelf life
  • Not suited for fine pitch

HASL is blown from the PCB surface to remove excess solder; this can create non-uniform coverage which makes component placement of tight pitch components difficult. The hot temperatures of lead-free HASL can cause warpage and soldermask embrittlement. The plated-through-hole may be plugged or reduced.

OSP—Organic Solderability Preservative:

  • Highest volume surface finish, worldwide
  • Applications range from low end to high-frequency server boards, also used in selective finishing
  • Latest versions are copper selective and more thermally resistant for high-temperature, no-lead applications
  • Applied through chemical absorption on the copper surface; no metal-to-metal displacement
  • Inexpensive surface finish
  • Limited shelf life

OSP does have implications at the assembly level. Older versions of this finish are not thermally resistant and couldn’t resist more than one reflow. The coating hardens with reflow exposure and becomes more difficult to solder. Material transfers onto the probe tip (during electrical test) can result in false readings and will require more frequent probe maintenance or a special probe style. Higher OSP thicknesses are detrimental to solder paste flow and hole fill.

Immersion Tin:

  • Applications are predominately automotive, U.S. military and aerospace
  • Excellent for press-fit applications (i.e., large back panels)
  • All contain anti-whiskering additives, but tin whisker elimination is not guaranteed.
  • Low cost, flat and suited for fine pitch
  • Aggressive on soldermask

Cautions at the assembly level include the fact that pure tin thickness is lost to the copper intermetallic with time and temperature. Loss of pure tin will degrade solder performance. The first reflow exposure will dramatically reduce the pure tin thickness and deposit stress could result in tin whiskers. This is a naturally occurring characteristic of tin in direct contact with copper.

Immersion Silver:

  • Greatest conductivity of all the surface finishes; well suited for high-frequency applications
  • Applications range from low end to high-reliability product
  • Topcoats have been formulated to overcome tarnish and corrosion issues in aggressive environments
  • Flat, suited for fine pitch with excellent solderability
  • Easily scratched, sliding connector limitations

The predominant issue seen at the OEM level is micro-voiding. Small voids occurring at the intermetallic layer of the solder joint could cause solder joint fracture. This defect manifests itself preferentially on solder mask defined pads which are more difficult to develop properly.

ENIG—Electroless Nickel Immersion Gold:

  • Highest revenue surface finish
  • Applications associated with high reliability
  • Used often in the flex market
  • Aluminum wire-bondable
  • No degradation between reflow cycles, can be held mid-assembly for extended times
  • New deposit thickness specifications for gold are under review to address the high cost of gold and hyper corrosion/black pad issues with extended dwell times for the gold

This chemistry requires tight process control. Proper plating conditions and control over the entire process are critical to performance. Proper chemical add-backs and numerous chemical analyses are required during start up and during plating. Layer thickness is also critical. Low nickel thickness will result in poor corrosion and thermal resistance in end use. Low gold thickness will result in less resistance to thermal conditioning during assembly and high gold thickness can promote nickel corrosion or black pad. Too much or not enough metal area in the plating bath will affect plating performance.

ENEPIG—Electroless Nickel Electroless Palladium Immersion Gold:

  • Gold and aluminum wire bonding
  • Applications include medical and U.S. military
  • Excellent solderability
  • Mitigation of black pad
  • Gaining interest and acceptance in the market

The primary caution at the assembly level is palladium thickness. Palladium that is too thick reduces the solderability performance. This will be slower to wet and have potentially palladium-rich areas in the solder joint. Palladium does not readily solubilize into the solder joint like silver or gold.

Surface Finish Breakdown by Market Sector:

  • Automotive: Silver, OSP, immersion tin
  • Data/Telecom: silver, OSP, ENIG
  • High end consumer: ENIG, silver, OSP
  • Low End Consumer: HASL, OSP
  • Aerospace, defense and high-performance electronics: HASL, immersion tin, ENIG, ENEPIG
  • Medical: ENIG, ENEPIG, silver

Regardless of whether your application is automotive, medical or military, there are many factors to consider when selecting a final surface finish. Cost, lead or lead-free requirements, end environment, shelf life, fine-pitch components, RF applications, probe-ability, thermal resistance and shock and drop resistance, to name a few. There is not a one-size-fits-all finish. Understanding the advantages and disadvantages of each surface finish allows the designer to select the finish that best fits each particular application.

Please contact us with any questions or for additional information!

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!

Rigid Flex: Total Cost Comparison

Rigid Flex: Total Cost Comparison

The transition to a rigid flex design from the traditional approach of using cable assemblies to join two or more PCB’s, has obvious benefits – space, weight, packaging, reliability and increased currently carrying capabilities – yet many times the perception that rigid flex is a high cost solution, causes designers and engineers to hesitate.   We are often asked for our thoughts on how to compare the cost of a rigid flex design with the more commonly used rigid PCB and cable technology. The answer is more complicated than simply comparing the bare board cost of the rigid PCB to the rigid flex. The rigid flex will almost always be more expensive. Reviewing the overall total cost of assembly for both approaches provides a more accurate comparison.

Recently, Elizabeth Foradori and I sat down to discuss this topic. Our discussion should not be taken as an all-inclusive list of items to be considered, every application is unique. Our hope is that this discussion helps facilitate the thought process when doing a comparison of the two technologies. To listen to the discussion, click here. Following are some of the highlights from that discussion.

Things to think about when comparing the total cost of an assembly:

Cost of design: You are merging multiple boards into one design, only one design is needed with a rigid flex. Often with a rigid PCB/cable solution, multiple PCB designs and multiple cable assembly designs are required. The costs for generating each design should be calculated and included when doing a cost comparison of both solutions.

Cost of cable and connectors: It is very common for someone to compare the cost of the rigid PCB’s with the rigid flex and come to the conclusion that rigid flex is too expensive. The cost of the cables and connectors should also be factored in. The rigid flex cost should be compared with the all of the components of the PCB/cable solution including: PCB’s, connectors, wire and cable, wire markers, shrink tubing, cable ties and fasteners and freight for all of these components.

Cost of the assembly operation: A rigid flex solution requires only one assembly while the PCB/cable solution can require two, three, or even more individual boards to be assembled.   The total cost of the assembly process needs to be considered when making the comparison. This will include items such as: the cost of kitting for assembly, labor, in process inspection, cable assembly test, final test, PCB tooling and test, and the costs associated with the engineering time required for each of these operations.

Cost of testing: Not only does the rigid flex solution only require one test operation, it also provides the ability to test the full assembly prior to installation.

 Cost of order processing: The costs associated with processing the order are very often overlooked. As stated earlier, the rigid flex is one unit, while the PCB/cable solution contains many different components to create the final unit. Each of these items has costs associated with purchase order generation, receiving and incoming inspection, material handling and storage, and payment processing.

Reliability: The rigid flex solution is considered a high reliability alternative to the PCB/cable solution. For many years, rigid flex was mostly associated with mil/aero applications, but is now becoming more common in nearly all markets. The flex connector becomes an integral part of the board; there are no solder connections between boards. With a rigid flex design, the reliability is dependent on a good design rather than dependent on the assembly operation

Logically, it is easy to agree that working with a single rigid flex design rather than the multiple components of a PCB/cable solution does simplify things. The big question becomes, does it save enough time and cost to justify the transition to rigid flex technology?

Every application is different and needs to be reviewed individually.  Following is a brief example:

Rigid Flex Cost Compare Example

As mentioned, this list is not intended to be all inclusive, but to provide a basis for further discussion when looking at comparing the cost of rigid flex technology with the PCB/cable solutions. Rigid flex technology is a growing aspect of the PCB market. As electronics becoming increasingly smaller, the space, weight and packaging (SWaP) benefits of rigid flex technology are more in demand.   Rather than simply comparing the cost of the individual rigid pcb’s with the cost of the rigid flex, analyzing the total cost of assembly for both solutions may enable designers and engineers to justify the transition to rigid flex technology to take advantage of these benefits.

Omni PCB

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: