Flex Material Handling: An inside peek

As more and more designs move to flexible materials to take advantage of space, weight or packaging benefits, it has been clear that flexible circuits require a different set of rules than their rigid counterparts. We spend quite a bit of time working through the design to make sure that the flex is as robust as possible. We also spend quite a bit of time on material selection, again to ensure that the flexible circuit withstands the flexing that will be required and performs properly in the end environment.

One thing we do not often talk about is what happens behind the scenes during the fabrication and assembly of the flexible circuit. What types of special handling considerations are in place throughout the manufacturing process to accommodate these thin materials? When you are auditing a potential new supplier, what things should you be asking about and looking for in their procedures?

Undoubtedly, the largest source of defects in flexible circuit manufacturing can be traced back to material handling. Drawing from my own knowledge and soliciting the expertise of several industry veterans  involved in flex circuit manufacturing—David Moody with Lenthor Engineering, Anaya Vardya with American Standard Circuits, Jim Barry with Eltek and Mike Vinson with Averatek—I have put together an insider’s view of the nuances involved in manufacturing flexible circuits.

Fabrication

Everyone agreed that it is the handling of the thin flexible materials that is the key to the successful manufacturing of flex and rigid-flex designs. A wrinkle, ding or dent in the copper material can easily, and will most likely, cause a defect. In fact, wrinkles are typically the leading cause of defects for trace and spaces errors in process imaging. So, what do fabricators do to mitigate this damage?

Skilled technicians are at the very top of the list. Thin, flexible materials require a unique set of processing parameters and significant time and effort is put into training operators on material handling. The movement of product between process steps is as critical as the precautions that need to be taken during each process.

When moving flexible materials between process steps, transport frames, slip sheets, and trays are required to provide the extra support needed to keep these panels absolutely flat—remember a ding or fold in the material will create a defect. When picking the material up for processing, consideration needs to be given to grasping the opposing corners to keep the panel flat.

Special consideration and handling is also needed when processing. Most equipment is not specifically set up to handle thin core, flexible materials. For example, moving product through the etching process or other conveyorized equipment requires “leader boards” or some type of frame to be taped to both ends of the sheet of flex material to provide stability and prevent the sheet from being caught up in the equipment rollers. If not done carefully, the process of applying the frame or leader and the subsequent process of removing the support structure is also an operation prone to damaging the thin materials.

Prior to wet processing, the panels are in full copper sheets. Once the excess copper has been removed to form the space and trace pattern, the panels are even more susceptible to handling damage. Care is given when creating the panel artwork to leave as much excess copper as possible on the panel. This could be the outer edges of the panel, the outer edges of each array, and between individual parts. It is not uncommon that the need for extra copper to add stability takes priority over the desire to maximize panel utilization.

Dimensionally, flex is far less stable than glass-reinforced rigid boards. The added copper in the panels also helps mitigate the material movement. This material movement creates unique challenges for registration both with coverlay application and in layer-to-layer registration for both multilayer flex and rigid-flex. Each manufacturer has a preferred method for registration and how they set up their tooling pin systems to best fit their processes.

Lamination is another area with unique requirements and special equipment for flex processing, including both lamination plates and specific lamination driver materials. Specialized materials are needed to fill air gaps and provide support through lamination. In the case of very thin core (.0005” polyimide) a base support layer may be needed.

More and more fine line flex circuits, particularly medical and sensor applications, are using extremely thin polyimide substrates with densities requiring additive processing rather than subtractive etch processing. These products are primarily double-sided with one side much more densely plated than the other, using both gold and copper to form traces on 0.0005” polyimide or thinner. Because of this, any plating stress will cause the parts to curl. For routing operations, UV-sensitive tape can be added to the panel to improve stability and support and improve handling.  This technology is similar to what is used in wafer processing. The parts will remain flat until the UV tape is removed. When removing the UV tape, the simple effort of being aware of the direction the material stress will cause curling, and then removing the tape by pulling against that direction, will help minimize the effect.

Assembly

Whether the flexible circuit has just a couple of components and is hand assembled, or the circuits are going to be run through a surface mount process, the number one thing that needs to be taken into consideration is the need to bake the material before subjecting the flexcircuits to high temperatures. Hand assembly is especially prone to defects with flexible materials. This process requires special consideration as to temperature and time and is yet another area that operator training is critical.

Flex circuits (and thin core boards in general) require some support mechanism to run through either wave solder or a reflow process. There are several options to accomplish this. A design with many FR stiffeners may move forward by using an FR4 carrier panel designed to provide stability to the array until assembly is complete and then have the excess FR4 removed, leaving the intended stiffeners. While that is one approach, it is more common to build the flex circuit as a single, individual piece and have a carrier fixture made to transport the circuit through the assembly line. This allows the fabricator to maximize the panel real estate and provide a lower cost unit price for the flex circuit. Carrier tooling is relatively inexpensive and is generally more than off-set by the lower cost flex circuit.

Flexible circuits are certainly a growing segment of the market and require not only special design and material consideration, but special handling throughout the manufacturing process. With material handling cited as the largest cause of yield loss during manufacturing, this is an area with (and for) continuous improvement. We all agree that employee training and on-going education is the key to success. Many facilities specialize in just flex and rigid-flex processing and others have teams dedicated to this product subset, but the common theme is knowledge and specialization. Flexible circuits often have a slightly longer manufacturing lead-time than their rigid counterparts and this is often related to the special handling and processing required for flex. Whether that is extra time in tooling and process planning, extra time during wet process, extra care needed to properly register the layers prone to material movement, or extra care needed during assembly, all special handling is done to maximize yield and provide a robust product to the end user.

Click here to read this blog in The PCB Magazine.

For additional information, contact Tara Dunn.  www.omnipcb.com

Final Surface Finish: How do you choose?

There are so many final surface finish options to choose from today, how do you decide which is best?  HASL – lead and lead –free, Immersion Tin, Immersion Silver, ENIG, OSP, and ENIPIG are the primary finishes used in PCB fabrication.  Fabricators and assemblers generally work with the majority of these surface finishes to be able to support their customer’s requirements.  So, the question is, with all of these available, how does an OEM select their preferred surface finish?

In the past, the primary function of the surface finish was to protect the copper from oxidation prior to the soldering of components.   Today’s expectations include:  superior solderability, contact performance, wire bondability, corrosion and thermal resistance, and extended end use life.  Designs have changed.  Lines and spaces are reduced, solder types and flux chemistries are different due to no lead requirements, the number of assembly cycles has increased and the product may need to carry high frequency signals.

Things to think about when selecting a final surface finish:

  • Does the application require lead or lead-free assembly?
  • Will the end environment have extreme temperatures or humidity concerns?
  • What shelf life is needed? Will it be months or years?
  • Volume and throughput
  • Does the design have fine pitch components?
  • How many assembly cycles will be required?
  • Is this a RF or high frequency application?
  • Will probeability be required for testing?
  • Is thermal resistance required?

Once the project requirements have been identified, the surface finish options can be reviewed to find the best fit.

Click here for detailed information for each surface finish.

Tara Dunn, Omni PCB

A glimpse into PCB Sales

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

Prospecting:

sales-quote-there-is-a-chance

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

When a customer calls:

thumbnail_sales-quote-there-is-a-problem

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

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

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

From PCB Users:

* A better than average knowledge of PCB construction

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

* Respond quickly when there is a request or issue

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

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

* Excellent communication skills

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

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

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

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

From PCB Manufacturers:

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

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

* Respond quickly and thoroughly

* Consistently find new opportunities and new business

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

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

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

From PCB Users:

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

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

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

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

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

From PCB Manufacturer’s:

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

* Stop relying on price to differentiate and win the order

* Close more business in a timely manner

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

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

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

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

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

Troubleshooting Flex Applications

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

UL Materials

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

Coverlay

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

Bend radius

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

Array configuration for assembly

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

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

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

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

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

 

PCB Sourcing? One Size Does NOT Fit All

When I am asked how to improve yields or reduce cost with a printed circuit board design, my mind immediately races ahead to the most common cost drivers: Has the part been designed with manufacturability in mind? Does the material selection make sense when balancing cost and performance? How many layers and lamination cycles are needed and could that number be reduced in any way? Has part size and panelization been considered? Are there any specific design features that push traditional design rules? All of these things have a direct impact on the manufacturers yield and the subsequent cost of the PCB.

One question that is rarely asked however is this:   how does a PCB sourcing strategy impact yields? I know, yields are typically associated with the manufacturer’s process capabilities and process controls as related to the printed circuit board design. But, let me pose a few questions to help shed light on the impact that PCB sourcing can have on manufacturing yields and subsequent profitability.

Have you ever wondered why it is so difficult to find a fabricator that can meet ALL of your needs? Wouldn’t it be great to find the perfect manufacturer, the one that has amazing service, does exactly what they say they are going to do AND has competitive pricing (total value, not just board price) across all the various technology levels?

The fact is, it is extremely rare for an OEM to have a homogeneous technology level across their entire PCB demand. On any given project, there may be a few 2-4 layer designs, a few 12 layer designs, a difficult motherboard design and maybe even a flex or rigid flex design.

It is also a fact that PCB fabricators have a “sweet spot” that best fits their equipment set, engineering expertise, facility size and company culture. Very often, browsing through a website or brochure will leave the impression that a manufacturer provides a “full range of technology”: 2 layer to 30 layer, .010” drill to microvias, standard materials to specialty materials, quick-turn prototype through volume production. At the end of the day, no fabricator wants to turn away business and they try to do their best to supply what their customers need.

BUT, there is always be a technology level, material set, or delivery window that each shop excels at. Their yields are maximized, the corporate culture embraces the technology and lead-time and ultimately prices are the most competitive. As an example, a supplier that excels at building 4-8 layer standard technology, likely runs with yields in the 97%+ range. But, if they were asked to build an 18 layer with blind and buried vias and via fill, the yields would drop dramatically. If the supplier that excels at building 18 layer blind and buried vias and via fill, with yields in the high 90% range was asked to build a rigid flex, yields would drop dramatically.

When sourcing PCB’s and creating a robust sourcing strategy, the challenge is identifying that “sweet spot” that maximizes a manufacturers yields and selecting the best group of suppliers to meet YOUR unique needs. Logically, if a circuit board is being sourced with the supplier that is the “best fit” for that specific technology level, their yields are going to be maximized, pricing will be its most competitive and ultimately profits will be increased for the OEM and the fabricator. While this sounds like a simple concept, the implementation of this strategy takes time and resources that are not always available.

Printed Circuit Board Sourcing Strategy, are you guilty?

Printed circuit boards are often one of the most expensive components of an assembly and arguably the most important due to their functionality and criticality. All too often, when time and resources are stretched too thin, these custom electronic components are purchased using the same strategy and structure as commodity items.

A PCB sourcing strategy might look like this:

  •  Treated as a commodity versus a custom component
  • Procurement strategy is often made at a tactical, not a strategic level
  • Many are doing business with suppliers without a full understanding of the technical capabilities, capacity or financial situations of their suppliers
  • Static strategies in a dynamic market – this market is changing rapidly
  • The same strategy is used for domestic and off shore sourcing. “One size fits all.”

This strategy can result in increased risk in terms of price stability and performance, increased risk of supply chain disruption and increased overall cost.

Do you need to revamp your PCB strategy? Where do you start?

You start with the basics. First review your PCB technology and volume requirements. Your requirements can then be segmented by attributes such as standard technology, HDI, heavy copper, flexible circuits, etc. Then search to match suppliers to these requirements. Audit the facilities. Don’t hesitate to ask the tough questions to REALLY understand the type of work each supplier excels at.

Next make sure that you have fully developed your procurement spec. Does it clearly spell out your requirements? Are any of your requirements adding unnecessary expense? It is not unusual to find that a corrective action implemented for an issue that happened 10 years ago is driving a requirement that increases cost and just isn’t necessary in today’s manufacturing environment.

Case study using strategic sourcing strategy:

Once a strong, diversified supplier matrix is put in place, analysis on large programs is simplified. To give an example, we were asked to assist with a pricing review and analysis of how to reduce cost on a specific project. This project included a set of PCB’s with a wide spread of technology. One design was a simple two layer design, another included microvias with copper via fill and there were three with technology between those two extremes. The volume required was expected to be between 1,000 and 5,000 pieces annually. We started first with the design for manufacturability questions and recommended adjustments where possible to increase yields at the manufacturer and ultimately reduce the total cost of the package.

After this, we looked at the current supply base and sourcing strategy. The decided upon approach was to select three suppliers, each with different technology specialties, gather pricing for the package and review the total package to determine the best path forward. The results of that exercise are included below.

Case Study Picture 2-9-16

Case study: Strategic Sourcing Review, project volume 1,000 to 5,000 annually

As you can see, the lowest price option for each part number is highlighted in yellow. From there, we reviewed the package from a single-source, dual-source or three-source perspective. The single-source option was ultimately the more expensive approach, with a three -source strategy providing the lowest cost option when looking at the PCB’s only.

With a savings of $16 per set over the two-source approach, the company can be expected to save between $16,000 and $80,000 per year on this project. From here, they can determine if the additional costs associated with managing three suppliers on this project is justified by the savings.

Conclusion:

When analyzing a set of PCB’s to improve yields and maximize profits, the first place to start is with a critical review of each PCB design. Are there any attributes that are pushing your manufacturers standard design rules? If so, is this necessary to the design or is there another approach that could improve the manufacture’s yields, reduce cost, and ultimately increase profit? Once the design is finalized, a critical review of the PCB sourcing strategy should be completed. Does the technology of each design fit the sweet spot of the selected fabricator? Would a multi-source strategy result in cost savings that justify the expense of managing more than one supplier on the project? Just as time and effort are spent reviewing and analyzing the design, time and effort should be spent reviewing and analyzing the subsequent sourcing strategy. Matching the technology to the best fit supplier will optimize manufacturing yields and reduce overall cost.

Contact us with questions or for additional information!  www.omnipcb.com

Copper Via-Fill Technology in Development

The use of via-in-pad technology is increasing rapidly in today’s printed circuit board designs. The need for miniaturization, combined with the rapidly decreasing pitch of component footprints, drives printed circuit board designers here. Via-in-pad requires the vias to be filled, planarized and then over-plated with copper. Once a designer has decided to move forward with this technology, the next question to be answered is what type of fill material should be specified. Typically, these vias are filled with either epoxy, conductive epoxy or solid copper plating. All have pros and cons to be considered.

I recently had the opportunity to speak with David Ciufo, Program Manager for Printed Circuit Board Technologies with Intrinsiq Materials, to learn about an exciting new product in development that will dramatically change the existing manufacturing parameters of the filled-copper via option.

Intrinsiq’s Nano Copper has been formulated into a screen printable paste that is compatible with commercial via-fill equipment. This paste can be dried and sintered in commercially available ovens and results in pure copper after sintering. The end product is highly conductive, both thermally and electrically, when sintered.

Benefits

Now, for the exciting part, there are two distinct advantages for PCB manufacturing with this product. First, because it is run with commercially available equipment, as seen in the process flow diagram, the capital investment needed to offer copper-filled via technology is significantly reduced. Many printed circuit board manufacturers are not able to offer the copper-filled via option due to the cost of plating equipment and chemistries. The barrier to entry for these PCB manufactures will be eliminated.

The second exciting benefit to this technology is the process time requirement. Solid copper-plated vias typically require 4 to 6 hours of plating time by the manufacturer, along with the specialized equipment and chemistry. This new product will enable PCB manufacturers to produce copper-filled vias in 60-90 minutes. A shortened cycle time will have benefits in lead-time and processing costs.

Via Fill Process Intrinsiq

Product release for this screen printable paste is currently scheduled for the end of 2016. Throughout this year, pilot programs will be released, further testing completed and reliability data gathered.

Product development, an interesting process

Nano copper inks and pastes are typically sintered photonically with broadband (xenon) flash or near IR laser. Because the copper cladding is too thermally conductive to allow complete sintering and high power lasers are a barrier to entry due to cost and complexity, an oven solution was sought to keep the process compatible with existing technology. Heller Industries manufacturers a formic acid environment convection oven to be used for flux-less reflow. This was determined to be the perfect environment to sinter nano copper without oxidation. Nano copper paste can be completely sintered in 40 minutes or less.

The process development for this product has had several iterations. The initial proof of concept was to deposit paste into mechanically drilled blind vias using a vacuum bag to help fill the holes. Those initial coupons were plated and etched prior to filling to allow for laser sintering. As the development progressed, the testing moved to copper clad PCB’s with mechanical blind vias. The panels were electroless copper plated then electroplated to simulate actual via filling requirements. Unfortunately, the thermal conductivity of the copper foil prevented the ability to sinter the copper paste. Research then pointed to thermal sintering in a formic acid environment.

As the development process continued, it was determined that the extended time necessary for formic acid sintering at 250C destroyed the PCB laminate. Moving forward, other nano additives were included in the formulation to lower the temperature requirement to 225C. This formulation and temperature sintered the vias completely in 60 minutes.

The next phase in the development process was to screen print trace patterns on FR-4 to be sintered alongside the via filled coupons. These samples were used to calculate bulk resistivity as compared to copper. Typical measurements were 6X to 8X that of bulk copper. Typical epoxy-based conductive via fills are in the 20X to 50X range.

Today’s product

Moving forward, additional product development was undertaken resulting in the current formulation, which allows the sintering temperature to be reduced to 190C. The paste is sintered to pure copper in only 40 minutes in the Heller conveyor oven. Samples of this formulation were via filled using the vacuum bag technique, on copper clad panels, with copper plated blind vias. The panels were Heller sintered, planarized, over-plated and solder floated. Samples were then subjected to IPC standard reliability testing parameters. Each sample was floated at 288C, held at temperature for 10 seconds, cooled, and refloated 4 times.   The vias survived 5 solder float procedures.

It is always exciting to learn about the new developments in products and processes for the PCB industry. In this case, incorporating nano copper inks and pastes into standard printed circuit board manufacturing techniques will allow manufacturers to offer a solid copper-via option to their customers without significant capital investment in specialized plating equipment.

Please contact us for more information.      http://www.omnipcb.com         tarad@omnipcb.com

 

Gold PCB Traces used in Medical Applications

Medical Research is Golden

Recently, I was involved in a group discussion about flexible circuits and the role of this product in medical equipment development and medical research. We were having a light-hearted discussion over lunch, when I was asked about the most interesting flex application I had been involved with. The first thing that sprang to mind was an application from several years ago. In this application, flex was being used for purely aesthetic reasons. A hand-held piece of surgical equipment included wires that were visible to the patient. The wires were functioning perfectly, but the negative perception of patients when seeing these wires during a medical procedure prompted the equipment designer to replace the wires with a sleek, high-tech looking flexible circuit. In terms of technology, this was probably one of the simplest flex designs to be manufactured: standard materials, single-sided, two big traces, and tolerances that weren’t particularly critical. Needless to say, the group was amused. Of all of the possible medical applications that I have had the opportunity to be involved with, THAT was the first one I thought of? Honestly, I have always appreciated that unusual application!

But, when giving that question more serious consideration, there truly has been a marked increase in flexible circuit designs in medical products over the past several years. Flex is the perfect solution for solving space, weight and packaging issues. A visit to the doctor’s office or hospital clearly reveals that medical equipment has become much smaller, lighter-weight and more portable, all while increasing functionality. Flex and rigid-flex designs are becoming commonplace in this field.   As we see an increase in the number of flexible circuit applications in this field, we also see an increasing need for finer lines and spaces, microvia technology and mixed material stack ups. This is not unlike the technology advancements we see with rigid printed circuit board technology.

Neural Probe Technology:

If I had to choose one of the most interesting flex applications that I have been involved with recently, it would be applications that involve neural probe technologies.   Designers working on research studies designed a sensor that required trace and space in the one mil range, which is not a simple technology to manufacture. Compounding the complexity of this unusual request was the need for those traces to be gold rather than copper. I did need to clarify that this was a need for gold traces, not copper traces with ENIG or gold plated traces! Wanting to learn more about the technology required to accomplish this combination, I reached out to Mike Vinson with Averatek Corp.

Averatek is a high tech company based in Santa Clara, CA that manufactures with a patented innovative and additive metal “print and plate” process. This additive technology enables the creation of trace and space widths below 10 microns and enables the direct deposition of copper and other metals on a variety of substrates.

One of the first questions I wanted to answer was: What would drive the need for gold traces rather than the traditional copper traces?  What I have learned is that neural probes are being used in many clinical settings for diagnosis of brain diseases such as seizures, epilepsy, migraines, Alzheimer’s and dementia. Microelectronic technologies are opening new and exciting avenues in neural sciences and brain machine interfaces. With this area of science and research, biocompatibility of the neural probes to minimize the immune response is critical. Copper, nickel and chromium can all adversely impact cells in the area of the electrodes. Flexible materials, such as polyimide, are commonly used in implanted devices to match the geometric and flexibility requirements of implants. Metalizing with gold provides further compatibility versus less noble conductors such as copper or nickel.

With a better understanding of the reasons behind the request for gold traces, the burning question was, how does the additive print and plate process enable both the fine lines and the gold metallization?

Fine Lines and Gold Metallization:

The traditional printed circuit board manufacturing process is accomplished by a subtractive etch process. The PCB manufacturer will start with a panel of copper-clad material. In other words, the full panel, often 18” x 24” is covered in copper. The traces and spaces are created with a “develop-etch-strip” process that essentially removes the unwanted copper from the panel leaving the desired trace patterns. Often over-simplified, this process is quite complex. After vias are drilled, electroless copper is deposited and resist is laminated prior to the photolithography process. Following the imaging process, panels are developed to remove resist that was not exposed, copper electroplated, and then tin is plated as a temporary etch resist. The remainder of the resist is stripped, the etch process removes the unwanted copper and the temporary tin plating is then stripped.

Additive technology is a reversal of this process. The manufacturer begins with the bare substrate. In the case of a neural probe, this is likely a polyimide material. The desired circuit pattern is then created by adding the metal layer to the substrate. Averatek has developed a proprietary nano-catalytic ink that enables a simplified five step process.

The bare substrate is prepared. Vias are drilled. The ink is coated and cured. The ink is then patterned with photolithographic imaging. Finally, metal is plated to this pattern. In this neural probe application, the metal is gold but metallization could also be copper or other metals. The key to this technology lies with the catalytic ink. Precursor catalysts that are deposited in thin atomic layers have unique properties like so many other nano materials. Additionally, a catalyst that is deposited via a liquid or “ink” can fill in many areas, nooks and crannies that would not be touched by line-of-sight methods like sputtering. This provides a basis for electroless plating that will fill vias of all types with more thorough coverage than conventional methods.

Their semi-additive process works by applying a very thin electroless metal to the base layer, followed by photo resist and imaging allowing the plating of a thicker electrolytic metal when required. As with a traditional semi-additive process, the resist is then stripped and the unneeded metal is etched away forming the trace pattern.   In the neuro probe example, when working with gold rather than copper, a very thin layer of conductive palladium is applied electrolessly followed by the gold plating. Gold is a difficult metal to etch, the palladium is easily removed without impacting the gold plating. The key difference when using Averatek’s catalytic ink technology is the ability to work with thinner metallization than the traditional semi-additive process.

Without the technology barriers associated with the traditional subtractive etch process, the additive process enables both fine lines and spaces (less than 1 mil) and very thin metallization (less than 5 micron).

Medical applications using this technology are often single- or double-sided configurations that have been designed with fine lines and spaces. The ability to design features less than .001” adds a new flexibility to maximize breakouts and eliminate, or minimize, multilayer blind via constructions. When this is coupled with the ability to plate pure gold without nickel, chrome, or exposed copper layers, a unique offering emerges for applications where the circuits may need to be exposed in end use.

This same technology has applications in other medical applications as well. Conductive layers are often used for shielding. In some cases, minimal thickness is required for bulk and flexibility. Utilizing the technology for a semi-additive base layer, as noted above, enables a very thin, yet very conductive metallization on flexible substrates as well as insulation on wires and cables for coaxial type shielding. This thin metalized layer can be cut to a specific size and installed around critical components in the final assembly. Many shielding applications require copper, but both gold and palladium can be used as required by the application.

Metallization of fabric is also an emerging market need. Using this additive technology, electrodes and other conductive paths can be formed by coating individual fibers in fabric, down to two microns in diameter, with thin metal layers. This has been demonstrated in gold, palladium and copper. The metalized surface can provide electrical, mechanical and chemical benefits.

Selecting the “most interesting” flex application related to medical field is not an easy task. There are just so many interesting applications and design developments to choose from. This field is moving at a rapid pace. At least for today, the neural probe technology, requiring both the traces to be metallized with gold instead of copper AND requiring those traces to be at or below .001”, gets my vote. Not only does it push outside of standard technology in one way, but in two ways, simultaneously. I thank Mike Vinson and Averatek for helping me learn more about the technology and processing required to meet these requirements.

Please contact me for additional information on this process.  www.omnipcb.com  tarad@omnipcb.com