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:

Flex-to-Fit, Flexible Circuits Solve Space Constraints

The “Flex-to-Fit” concept reminds us that creativity and engineering go hand-in-hand.  Click HERE for the video recording of our Fit-to-Flex discussion.

Imagine this scenario: As an engineer, you have been tasked with the challenge of adding sensors to the front spoiler lip of the new 2015 Porsche Cayman. There is limited space available and the cavity is thin enough that running even a small wire bundle would be difficult.

What do you do? Let’s take a look at the Flex-to Fit concept.

When there is not ample space for a conventional approach, this process, which is the convergence of the mechanical world and the electronics world, results in the ability to design a flexible circuit along the contour of an existing, irregularly shaped structure. By taking the mechanical part, extruding the surface and then conforming to that surface, a flex circuit can be created that will fit perfectly within the confines of a limited space or cavity. After talking with Mike Brown of Interconnect Design Solutions, he helped to clarify this process, discussed several exciting applications and explained the benefits to the flexible circuit design process.

Most electronic systems require an enclosure to support a rigid printed circuit board. Looking beyond the constraints of an enclosure and incorporating flexible circuits within the contours of other existing structures, opens up endless possibilities. In the example above, imagine this solution; the valence of the front spoiler lip is mechanically digitized and recreated in a 3D MCAD model. The surface is then lifted and flattened into a mechanical piece and translated to the ECAD environment to layout the flexible circuit.   The flexible circuit is then designed to conform to the exact contour of this irregular shape.   Sensors running along the flex circuit solve this challenge of limited space with the added benefit of reducing the weight.

We are in a time of amazing developments in our electronics products. Today’s electronics are increasingly smaller, faster, lower power, lighter weight and feature rich. Flexible circuits are commonly used to replace wire bundles to reduce size, weight and power (SWaP). It is also common to use a flexible circuit when space is confined and circuitry is needed to be folded around corners and into tighter packaging.   When traditional solutions no longer meet design constraints, the Flex-to-Fit model allows us an alternative path forward. As we step back and look at the existing structures available with a creative eye, it can be both exciting and a bit daunting. Imagination and analytics often compete and the combination of both is needed to determine how a space can be best utilized.

Extruding the surface of irregular shapes and creating a perfectly fit flexible circuit to integrate into the contour of that structure opens up so many possibilities. Thinking “outside the box” can save space, weight, cost and promote ease of assembly. The applications for this approach are endless. For any product in the automotive, aerospace, military and commercial sectors, where restricted weight and space are major factors, Flex-to-Fit offers excellent solutions.

Imagine another example; if you were to extrude the internal surface structure of a wing or fuselage of a drone or autonomous vehicle, the flex circuit could be modeled to fit the exact contour of the area it is to occupy. The cavity that would otherwise be consumed by bulky wiring cables could be made free to accommodate more features, whether it be additional sensors, monitoring or enhanced functionality.

One last example is a product that is hot in today’s market, wearable electronics. Rather than run a bunch of wires and all of the sensors in a shirt, which can be a bit bulky, one possibility is to sew in flex circuits that have been modeled or molded around the human body. The flex can be sewn between the layers of material resulting in a smoother surface more closely resembling regular clothing.

While talking with Mike, it was easy to see the possibilities and the benefits to the end product. It is also important to discuss the benefits of this process to the flexible circuit design itself. By extracting the exact contour of the part, flattening it, and transferring this to the ECAD design tools, the designer is able to accurately analyze the flexible circuit design in the ECAD model.   Often when using a flexible circuit in an unusually shaped area, the added length required and bend areas are difficult to determine. This approach allows the designer to perfectly fit the flex to the structure it will be aligned with.   The designer is also able to accurately analyze the proper bend radius and make adjustments to remove copper layers or adhesive layers to meet standard design rules. Stiffeners and cut out areas are also able to be analyzed directly in the ECAD system. Because all of these items can be reviewed to the exact fit of the piece, the end result is a more accurate design. There will be no surprises as the piece is assembled in the unit and this can potentially reduce the number of revisions during the design cycle.

To identify a structure that is not being utilized, digitally scribe that structure to create a MCAD model, flatten the surface of that model and transfer that to the ECAD system for flex circuit design clearly demonstrates the convergence of the mechanical world with the electrical world. The convergence of these two disciplines brings so many new opportunities for today’s electronics. Applications for the Flex-to-Fit concept are really only limited by our creativity and imagination. It is an exciting time to be involved in the world of flexible circuit design and manufacturing.

Please contact us for additional information!

Tara Dunn, Omni PCB   and  Mike Brown, IDS

Is it “Just a Board”?

I was out with friends one night, a table full of people holding many different conversations at one time. I clearly hear the words, “but it is just a board”.   The background noise dimmed and I suddenly became laser focused on that particular conversation.   I felt an adrenaline rush and the unstoppable need to defend the product I have chosen as my area of expertise.   I took a deep breath and calmly asked, “Why do you say that?” The result was a lively discussion about the function of the PCB in today’s electronics.

In fairness, this person’s background is in the component design side of the industry and his limited experience with PCB’s involved 2 and 4 layer, standard technology designs. So, yes, I get where he was coming from. You can buy a simple PCB at most shops and have good quality product. BUT, today’s electronics require the PCB to be so much more!

We are in a time of amazing developments in our electronics products. Electronics are required to be increasingly smaller, faster, lower power, lighter weight and feature rich. As consumers we can all appreciate this. The primary function of the PCB, other than being a solid base for components is to provide the interconnect between the components that are accomplishing these things.

Electronics today push PCB designs well past “standard technology”: specialty materials, finer lines and traces, microvias, both stacked and staggered, multiple lamination cycles, heat transfer, impedance matching, electromagnetic shielding, embedded components, etc. The phrase, “it is just a board”, just doesn’t apply.

PCB fabricators are continually developing new processes, pushing their technology limits and tightening process controls to meet these requirements. PCB designers need to understand the new materials, manufacturability constraints and cost drivers. The electrical, mechanical and fab people working together can create amazing things.

We rarely use this format to “get on our soap box”, but we are really curious……what does everyone think?

Is the PCB, “just a board”, or is it a critical aspect in the electronic assembly?

Send me a note and let me know you thoughts!

Do you avoid rigid flex design?

It is common to hear, “we avoid rigid flex”,  with the most common objection often being the learning curve to produce a good layout.  Today’s Quick Tip lists the benefits of using a rigid flex construction and situations where this technology makes good sense.

Rigid flex circuits are a hybrid construction consisting of rigid and flexible substrates laminated together into a single package.  They are electrically interconnected by means of plated-thru holes and can be solid flexible or loose leaf flexible construction,  with or without a stiffener.

When to use Rigid Flex vs. Flex

  • When stable area is needed for component mounting and packaging requires flex to fit or flex to install.
  • Used when components are mounted on both sides of the rigid and flex section.
  • Used to solve high-density packaging problems.
  • EMI/RFQ Shielding.
  • Dense Surface Mount Assembly.
  • Controlled impedance with shielding applications.
  • Used to connect rigid boards together.

Benefits of Rigid Flex:

  • Rigid-flex circuits offer enormous advantages in quality, especially with high vibration applications – eliminating connectors, mis-wiring and reducing assembly process steps.
  • Reliability of the assembly is proportionally increased due to the reduction of solder joints.
  • Weight reductions, due to the elimination of connectors and solder joints.
  • The performance of a rigid-flex is dramatically superior to a similar design with the rigid PCB’s and jumpers.  The connector leads and through holes required to join the jumper to the rigid PCB add parasitic inductance and capacitance to the circuitry.  The inductance of one net of the soldered jumper is in excess of 1.5nH, vs 0 of the same net in a rigid flex solution.  Speed, power and clarity of the signal is degraded by the use of the jumper/connector assembly.

Rigid-flex can be differentiated from multi-layer flex construction with stiffeners by having conductors on the rigid layers.  Plated thru holes extend through both the flexible and the rigid areas, with the exception of the blind and buried via construction.

We always recommend involving your supplier in the early stages of the flex or rigid flex design.  An experienced flex circuit engineer 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 if you have any questions or would like additional information! 

Remember, designing and purchasing printed circuit boards does not have to be difficult!

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!