Tag Archives: flex

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!

 

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

www.omnipcb.com

Polyimide Coverlay and Adhesive Squeezout

When a flexible circuit requires high dielectric or dynamic flexing, an adhesive coverlay film is often the best choice.

This coverlay film is traditionally a layer of adhesive bonded to a layer of polyimide. During processing, heat and pressure are applied to the stack up causing the adhesive to soften and flow.   The adhesive will flow (squeeze-out) slightly beyond the coverlay openings.

This process is necessary for complete encapsulation of the coverlay and to protect the edges of the film from chemicals or abrasion which might cause delamination.

Although this is a desirable result of bonding the coverlay, this “adhesive squeeze-out” also reduces the solderable area of the coverlay opening, and must be accounted for in the design stage.

We are often asked what an acceptable amount of adhesive squeeze-out is. According to IPC-A-600, the coverlay coverage shall have the same requirements as the soldermask coverage in rigid printed circuit boards. The acceptability requirements for coverlay coverage include both the coverlay and the squeeze out of adhesive and are different based on which Class is being built to.

For example, Class 3 requires 0.05 mm (0.00197”) solderable annular ring for 360 degrees of the circumference. Class 2 requires this same solderable annular ring for 270 degrees of the circumference and Class 1 requires a solderable annular ring for 270 degrees of the circumference.

We always recommend involving your supplier in the early stages of the flexible circuit design. An experienced flex circuit engineering will be able to guide you to the correct material stack up and tolerances needed to ensure you receive the product you require.

Please contact us for additional information.  Designing printed circuit boards should not be difficult! 

www.omnipcb.com