“How to Design with Flex in Mind” Webcast
Question and Answer Session
Flexible circuits are growing in popularity and Minco has considerable experience and expertise in its various applications. To help design engineers become more knowledgeable, Minco conducted a webcast titled, “How to Design with Flex in Mind,” which provides information on specific design and material dynamics.
Immediately following the webcast was a live “Question and Answer” forum between design engineers and Mark Finstad, Principle Applications Engineer for the Flex Circuit Division at Minco.
Below is a transcript of that session.
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How many times can we bend the same circuit with out cracking it?
That depends a lot on the on the particular application. Obviously, a thicker circuit is going to be able to tolerate multiple bending operations a lot less than a thinner circuit. So, if you have a single layer circuit, it is a function of the bend radius and how many cycles you are planning on flexing it. Typically with multi layers, the IPC recommends that once you form it, you don’t move it. It should stay in that configuration, and it is a good idea to constrain the circuit after it is formed so any additional handling around the assembly does not exercise that bend at all. For a single or a two layer circuit you can typically move it a little bit with out having a problem unless it is an extremely tight bend radius where you are getting down to a 3:1 or 4:1 bend ratio.
Are flex circuits available with Teflon combustion safe materials?
You can use Teflon as an insulation material, usually on the outer layers. It can also be used internally as an adhesive. It is not something that is mainstream technology right now, but is being done. Teflon has been used as a cover material for 15 to 20 years now. But as an adhesive it is not used quite as much.
Is it common to provide integral temp sensors in a heater circuit?
Yes, Minco also has a division that manufactures temperature sensors and heaters, and we regularly put integral temperature sensors into the heater in order to monitor the temperature of the heat sink.
I need to form a circuit with component installed. Will the form withstand a reflow process?
Typically after you form a circuit, you don’t want to subject it to any elevated temperature. There a couple ways you can approach it. You can install all of the components, and then form it after the components are installed. This will require some pretty elaborate tooling to provide clearance for those components so you do not damage anything when you are forming it. The other option would be to do the forming first, before the components are populated on the board, and then solder the components by hand.
You mentioned that certain applications will allow a safe bend radius lower the IPC guidelines. How do I know if my application can go below IPC guidelines?
The best thing to do in this situation is to contact a manufacturer since every application is different. Single sided circuits that are built properly can go down to about a 3:1 bend ratio with out having any problems as long as it is constrained after it is formed. I have found that even multi-layer circuits can go down into around 5:1 provided there are no discontinuities in the bend area. Having everything very uniform will allow the circuit to be bent considerably sharper than the IPC guidelines recommend, without having any problems. When the IPC guidelines were being written they had to be intentionally conservative because there are so many different variations of flex circuitry. It was important that the guidelines be laid out so they would work for any application.
How does the cost of a flexible circuit compare to a wiring harness?
When comparing the raw cost of a flex circuit vs. a wiring harness, the flex circuit will be slightly more expensive for just the component. If you look at the total cost of ownership, typically the flex circuit will be less expensive. It also has a lot to do with the number of wires in the wire harness. If your wire harness only has three or four wires, the flex circuit may not be a good option. If you have 50 to a 100 wires it can definitely be less expensive both from the cost of flex and the total cost of ownership. One nice thing about a flex circuit versus a wire harness is that there will never be a wiring error on a flex circuit. Every flex circuit is photo defined from artwork, and every one will be exactly the same, and they all install exactly the same. You will never have a situation where a wire harness has a crossed wire, gets installed in an application, and it is not found until later.
What would be regarded as disadvantages and advantages between acrylic adhesive and All-Polyimide (AP) film for the type of materials to use in a flex circuit?
Acrylic adhesive is pretty much the flex circuit industry mainstay for bonding circuits together. It has been used for over 30 years. The AP construction is typically used for very high temperature applications, which is really the only reason you would use an AP construction. In a flex circuit, the acrylic adhesive is typically the weak link. The thermal properties of the acrylic adhesive are inferior to anything else in the stack. Acrylic adhesive does beat the polyimide in typically every other category such as bond strength and in flexibility, so you have to evaluate your application. If it is a high temperature application then you probably have to go with an AP construction. If you go with an acrylic adhesive, the temperature rating will be lower but your costs will also be lower.
How well do flex circuits take wide temperature variations, say -40 to 125°, compared to conventional circuit boards. Is the life expectation of a flex circuit in harsh environments shorter than a conventional PCB?
It depends if it is Fahrenheit or Celsius. If that is a Fahrenheit number, the flex will last indefinitely. That is well within the temperature range of all the materials. At extremely cold temperatures, the flex will be more prone to damage if it is moved or vibrated. Obviously, if you get down to extremely low temperatures, the materials can become brittle. A hard board is not expected to move, so you don’t have to worry about that. At elevated temperatures, up to about 250°F, you really don’t have any problems. If you start going beyond that it is the function of the actual temperature and the time it is exposed to that temperature. As the temperature goes up, the flex circuit can be exposed to that temperature for less time before you start having a problem. You can typically take a flex circuit up to 350°F for a short period of time, maybe even 375ºF for very short period of time, if it is a few seconds. If you run it under 250ºF it will run indefinitely with out any problems.
When forming bends with a fixture, is heat required or should the bend be formed at room temperature?
It can be either one. It depends on how the form has to be accomplished. If it is a very thick circuit, some heat can soften the acrylic adhesive, and it allows the circuit to be formed a little bit easier. A cold form will typically have a little bit more memory. If you were to “heat form” a circuit and then constrain it and cool it, it will hold that form extremely well. If you “cold form” the flex, there will be some memory, so you will have to over form a cold form to get the final configuration that you want. Depending on the bend radius, over forming may be a problem. If you are getting down into that area where you are concerned about the bend ratio you may not want to over form it and heat may be the way to go.
What is the cost adder when using adhesiveless design base material?
Virtually, nothing. The material processing is exactly the same for the manufacturer as adhesive based material so there is really nothing as far as the processing cost. Several years ago, the material itself was considerably more expensive, but it has come down to where its only about 20% difference in material costs. But considering that material costs makes up only 15-20% of the overall circuit cost, it really is negligible. So typically if you have a multi-layer or rigid-flex circuit, you are going to want to go into adhesiveless material because the negligible cost adder is more than out weighed by the performance you will get at higher temperatures.
Does a Liquid Photo-Imageable Coverlay (LPI or PIC) cover layer allow tighter bends?
It depends on what type of photo image cover lay you are using. Some of them are fairly brittle and you will get worse performance than you would with a Kapton cover. Some are more flexible. Typically, the best flexibility we find comes from using a standard acrylic and Polyimide cover.
I have a double layer circuit that I want to have bent to a 90 degree angle and after installation the circuit will never see any deviation from the 90 degrees. I would like the bend pre-formed at the factory. Does the pre-forming at the factory and/or the lack of bending after installation allow me to go lower then the 10:1 ratio?
For a two layer circuit, absolutely. If you can guarantee it is not going to move after it is installed and there is not going to be any exercise of that joint during installation - because that can cause problems also. If it is being handled and things are being plugged and unplugged and the joint is being exercised, it can cause problems. If it can be constrained so it is not going to be moved, I think you can safely go down to a 4:1 or 5:1 area and not have problems. Another thing to take into account - on a two layer circuit you are going to have plating on the outer layers, and there is going to be some variation from vendor to vendor. You are going to want to check with the manufacturer that is making the flex circuit to determine what kind of elongation and tensile strength they have on their plated copper. If they are running less than 15-20% elongation on their plated copper, you may not be able to go down to those ratios that 5:1 or 4:1. If they are getting the 15-20%, you should be able to go down to those ratios with no problem.
Can flex circuits be used for carry power traces as high as 20 amps?
Sure, you just use heavier copper and make your conductors wider. Another thing to do is to keep your power carrying conductors on your outer layers. There are quite a few different places you can find the information on that. Minco’s Flex Circuits Design Guide does have a nomograph that will give you the current carrying capacity, and the width of the conductor, and the weight of the copper that can handle that current. Twenty amps are probably off the charts but you can extrapolate where it would be based on the copper weight. Naturally, the heavier the copper, the more current that it can carry. And like I said, you want to keep those current carrying conductors on the outer layers so they can dissipate the heat the easiest. If you put them on the inside, you are going to have to go with a wider conductor to eliminate the heat being generated in the conductor itself.
How close to the bend area can a through via be placed? Also, if a line of vias exist, should they be a staggered across the width?
If the vias are not in the bend area it really doesn’t matter where you put them. They can be scattered pretty much anywhere. Lining them all up can cause a problem if you are trying to bend it close to that area because the vias are going to cause a discontinuity in the construction, and it may actually want to pull the bend into that area even if you do not want it to bend there. As far as distance away from the bend, if you stay 100 to 150 mils from where the bend ends you shouldn’t have any problems.
What is the tightest manufacturing tolerance available on controlled impedance flex that you are aware of?
Pretty much everyone likes +- 10%. What drives the impedances on a flex circuit is the conductor width and the thickness of the materials. We maintain extremely tight control on the conductor and the material thickness between the conductors. All the insulation materials are extremely accurate as far as tolerance on the thickness. The only variation is in the etch tolerance. The etch tolerance is driven by the weight of the copper. If you go to one-half ounce copper, you can get tighter etched tolerance than if you have one ounce or two ounce copper. So it is kind of a sliding scale. Most manufacturers will guarantee +- 10%, and you may be able to twist their arm and get down to 7 or 8%. Once you get down to +- 5%, there will be a cost adder attached to the circuits because the manufacturers know there will be some variation, and they will be throwing stuff in the garbage. If they are going in the scrap barrel instead of a shipping envelope, it will add to the costs of the ones that do go out the door.
Is one-half ounce RA copper more or less flexible than one ounce RA copper?
I prefer one ounce. One-half ounce is a little bit thinner and you can get the overall thickness of the circuit down a bit. My experience has been that one ounce copper is more robust and holds up a lot better. The change in thickness is really kind of negligible, less than a mil in overall thickness. I prefer a little more weight in the copper to give you a little tougher conductor.
What features drive higher costs?
The really big one is circuit size, because most circuits are panel based. That is how most manufacturers build them. The more circuits that are put on a panel, the less you pay per circuit. Layer count is another huge driver. The layer count and the circuit size are the two biggest cost drivers. Another cost driver would be feature size. As you start getting smaller pads, and when you get into multi-layers, the cost will increase. Etched features are usually not a problem until you get under four mils for line width and spacing, but if you get below that it can also become a driver. Really, the big ones are circuit size, layer count, and some of the feature sizes.
How close to the beginning of the bend can the rigid PCB portion of the flex circuit be for a four layer flex circuit?
I assume we are talking about four flexible layers not four total layers. Assuming this is a rigid flex circuit, you are going to want to put some kind of strain relief along your interface, and the flex circuit manufacturer will need some tolerance for that strain relief. Typically that will eat up at least 100 mils and then you should allow another 100 mils beyond that. So that puts you about 200mils from the rigid interface.
What are your thoughts on the flexibility of silver filled epoxy shield layers at temperatures of -130 to 50°C Mars environment?
The higher temperature is not going to be a problem. I don’t believe that the silver epoxy manufacturers have any data going down to the lower temperature. Typically they go down to about -40 or -50°C and that is where they stop with their data. Obviously, with epoxy being a plastic, it is going to become some what brittle when the temp drops that low. Guessing at the extreme temperature you are talking about, -130°C, there may be some problems with the epoxy starting to get brittle. You will also want to check the acrylic adhesive because that is going down lower than acrylic is rated to also.
Can you use standard through-hole connectors for flex circuits? Or are there special connectors to be used, and if so can you name a manufacturer?
You can put nearly any connector on a flex circuit that you can put on a rigid PCB. What you would normally do for a through-hole connector, is on your flex circuit you would put a rigidized backing on the side of the circuit that the connector will be mounted on which leaves solder access on the other side. There are a lot of connector manufacturers out there, pretty much anyone who supplies to the rigid industry will also be supplying to the flex industry. A couple I can recommend would be Omnetics and Hirose. We use both on a regular basis.
Can pre-forming a flex bend be done to alleviate some of these issues?
Pre-forming can absolutely be done by the manufacture. We routinely do forming for customers if they do not feel comfortable doing it. We also offer forming tooling that we manufacture here, that allows the customers to do the form themselves but with tools that are made specifically for the circuits they are forming. It depends on the application. If it is a fairly large circuit and the 3D finished shape ends up being large like a cubic foot, pre-forming it at the manufacture may not be an option. It depends on how big your circuit is and how big it is after it is formed. But you can get custom trays that will constrain the circuit in a formed condition for shipping and we do use those on a regular basis also.
You mentioned minimum bend radii pre-forming. Do the same rules apply for flex cycling?
No, by flex cycling I assume you are referring to a dynamic flex application where the circuit is going to be exercised on a regular basis. The bend ratio has to be more liberal than what I was talking about. In the webcast, I was talking about static applications where you bend it, form it, install it, and it stays there. If it is going to be routinely bent or flexed, you are going to have to have a greater bend radius. There are different levels of static applications; you have static where it is bent once and not moved, semi-static where during servicing it may be exercised a little bit, and dynamic static where it is moved on a regular basis. For dynamic applications you have to make sure the overall stack of materials is extremely balanced. Your conductor layer has to be perfectly centered and you have to have exactly the same type and thickness of materials both top and bottom so the bend axis falls right down the center of that copper conductor. Also for dynamic applications you have to limit it to a single layer. On any more than one layer, if you look at a cross section, the outer copper will be to the outside of that neutral axis and inner copper is going to be in the inside, and you will work the copper and over time it will start to harden and you will end up with broken conductors.
What material choices typically exist plating the bare copper traces as they exist in a window? Is it possible to do plating with exotic materials?
The most common plating is copper plating which we use in a panel based operation. When talking about a window I assume you mean a final finish. Some of the most common ones are gold and ENIG. We also do a lot of silver plating. These are the most common finishes.
You mentioned bending, but can a circuit also be twisted, say 45 degrees following the Z axis.
Yes, you can twist a flex, but it also depends on the application. The thicker the circuit is, the more you are going to have to elongate that twist. You can not grab it in two places a quarter inch apart and bend it because you would sheer the circuit. The thicker the circuit, the longer the area you are going to have to allow to make that twist. But if you can do it with your fingers, you will probably not hurt the circuit. If you are doing it with some tooling where you are grabbing it with a vise grip or similar, and start twisting on it, you can certainly do some damage. Typically with finger forming you will not have a problem.
Is the bend radius measured at the neutral axis?
We typically measure the bend radius on the inside of the bend, around an imaginary mandrel you would bending the circuit around.
What are the pros and cons of using a flex circuit over a standard FR4 based printed circuit board?
They are flexible, and that is really the biggest advantage. A rigid PCB is going to be considerable less expensive, so if your board does not have to operate in more than just the X and Y axis, you are going to want to go with a hardboard. Flex circuits are used when you need to link several hard boards together to make the interconnect and you don’t want bulky board to board connectors, and you don’t want wire harnesses. A common use for flex circuitry is to get connections from your main PCB board up to things like switches and connectors on the side of a panel. So typically if you are working in more than two axis, that is where the flex circuitry comes in.
Does your company offer a design guide?
Yes. Go to Flex Circuits Design Guide. Another piece of literature you may find helpful is “Designing for Flexibility and Reliability.” They are both available for free download.
What are your general thoughts or cautions on the differences between acrylic and FEP adhesives for controlled impedance cables at low temperatures of -130°C?
The biggest concern would be the adhesive starting to get brittle. I could not tell you off the top of my head what the lower limit of Teflon is, but I know that for acrylic adhesive, -130°C is starting to get below its temperature ratings. You are getting into a gray area here. It is not a bad idea if you can simulate the environment and contact a manufacturer who can provide some samples that you can test. Manufacturers always have reject circuits laying around of the construction you are looking for. These can be sent out if you so you can simulate the environment and run them through their cycles. You can probably tell which construction will work best for you after testing.
Could the track material be another metal besides copper?
Absolutely. Another division of Minco makes flexible heaters, and they use Inconel and cupro nickel as two of the most common metals in those applications because they heat up when current is run through them. Copper, with its low resistivity, is best for just making interconnects. So for making flex circuits, the most common one is copper, but we do use other materials if we want to generate heat.
What is the definition of “heavy copper conductor?”
A Heavy copper conductor I would say is anything over two ounces. One-half ounce, one ounce, and two ounces are the most common copper conductor weights that are used and I would not consider any of those heavy. When you start getting over two ounces, I start considering it heavy. You can go all the way up to five to ten mil thick copper, but the draw back is at a certain point it is no longer a flex circuit. If the copper gets too thick it doesn’t bend and you are not really getting any flexibility out of it. It is kind of a balancing act trying to figure out how much copper you need to carry the current, but still get the flexibility you need. If it carries the current, but does not bend, it doesn’t do you any good.
Are there any perforation guidelines for a top and bottom shield plane solid grid?
I think you are talking about cross hatched shields. What I have found is that cross hatching really doesn’t have a huge impact on the flexibility. There are some advantages to doing that, but the only thing you are really reducing is the copper area. The adhesive and insulation thickness doesn’t change, and that is driving the thickness of the circuit. The other thing to keep in mind is that if those planes are there for impendence control, and you are using them for return paths, then cross hatching your plane will have an impact on the impedance. When the copper is removed from those plane areas you are creating some discontinuity for the return path, because you will reduce the capacitive coupling and will drive your impedance higher. It is important to let a good designer scrutinize the design. Make sure that the conductors are being run in such a way that you are not going to end up with whole bunch of reflections from cross hatching. You don’t want to be doing things to increase your mechanical performance if it is going to have detrimental effects on the electrical performance.
If I pull my stitching vias from bend areas, won’t I have EMI problems in those areas?
From our experience there really isn’t a big advantage to having the stitching vias because our dielectrics are so thin. Typically, if you have stitching vias around the perimeter, you also have a guard trace that is running around the perimeter. That guard trace by itself pushes all of the noisy or quiet conductors inboard far enough where there really isn’t going to be any EMI going in or out regardless of whether you have stitching vias. Some designers feel more comfortable having them there, and it is not a huge cost adder putting the holes in, so we do not spend a lot of time convincing people not to put them in. However, in the bend areas we recommend that they are removed, and we have not seen any electrical performance problems associated with that.
Can a heater and temperature sensor be imbedded inside the rigid portion of a rigid flex design?
Yes, you would use a resistive metal. It is not something that would be considered “run of the mill,” but it is something we have done so it is possible. Keep in mind that if you are burying it inside a rigid portion, it will be harder to get that heat out. You are going to want the manufacturer to look at the application to determine what you are trying to accomplish. If you have a couple of 30 or 40 mil hard boards over the heater, it will take a lot of heat before it starts radiating to the outside surface of the part. So there maybe some special features they want to add to get that heat closer to the outside surface.
Regarding solderable finish for a BGA assembly - is OSP a good candidate? Why or why not?
We have not seen a huge amount of OSP, but we have used it a little bit. The most common thing we are doing right now is ENIG, and then the component placement vendors will typically solder paste, and put the BGAs on that way.
Where is the bend radius measured for unbounded flex circuit?
The bend radius will be wherever the center of the bend is. You also have to look at a secondary bend radius where the puckering is. Typically you want to make sure that puckering is not dropping that bend radius, and becoming a limiter. You have to look at the application, how tight you are trying to bend it, and how much distance there is between the rigid areas. We typically measure the bend radius at the actual bend itself, and make sure the associated puckering doesn’t cause a secondary bend radius concern.
Are there any tougher materials than pure copper?
Well, there are many materials that are tougher. Inconel for instance is a very hard metal. However, you are going to get a lot of resistance, and you are going to basically make a heater if you put any kind of current through it at all. Really from a toughness standpoint, rolled annealed copper is probably your best bet because it is soft and has high elongation. It can stand an awful lot before it breaks. Keep track of what you are doing with the circuit before and while you install it, and make sure that you are not exercising it beyond its limits.
Are there instances where you would not want to put a strain relief fillet on a flex with stiffener?
I would say that if you are not bending it near the stiffener, it is not necessary. It would just be a cost adder. If your stiffener is extremely thin, the strain relief fillet could actually be detrimental. If you have a five mil thick stiffener on your circuit, there is no way you can put a five mil thick fillet. What happens is that you end up putting on a semi-cylindrical bead, and you replace one straight edge with another. I would say you have to look at the individual application. If you have a thick hard board, and you are bending it close to that hard board, you should absolutely put a strain relief bead there. If you are not bending it anywhere in that area, or you have a cost sensitive application, you may want to leave it off. Again, your manufacture can guide you, if you tell them what you are doing with the circuit. The manufacturer can tell you where you need the strain relief bead.
NASA Mars rovers use four layer dynamic Polyimide acrylic flex cables at -130°C for two and half years. Would you recommend an alternate construction?
A: It sounds like they are doing a pretty good job at that. As I mentioned earlier, it goes down below the temp limits that the manufacture of materials specifies. That is why I recommended getting some material or circuits and testing them. It sounds like they were able to simulate the environment by putting them on Mars.
Question and Answers after the webcast
My single layer flex circuit has many sharp folds. Does the angle of the fold line with respect to the trace lines affect the stress on the trace?
Single layer flex circuits are pretty resilient due to how thin they are. Also, the copper tends to be right on the neutral bend axis on most single layer designs. For this reason, you can probably get away with routing conductors through the bend areas at angles other than 90 degrees without experiencing any problems. I would recommend that you attempt to constrain the bends after you have formed them with PSA or similar material. If the circuits are formed, and not exercised, they should be fine.
Different coverlay materials?
By far the most common coverlay material is polyimide (i.e. Kapton, Apical). Polyimide covers are the standard on well over 90% of all flex circuits. Photo-imagable coverlay (PIC) has been gaining popularity in recent years due to technical advances in the PIC materials, and the ability to have square and rectangular access openings. We have also used Teflon as a coverlay material on a number of different designs. There are probably other materials that could be used, but you will not find a flex circuit vendor that will have them on their shelves. This means special orders, long lead times, and many times minimum buys which are passed on to the customer.
Does this forming tool have a sharp edge?
No! You should never use a tool with a sharp edge on an flex circuit. It would be very easy to cut the polyimide insulation with a sharp metal edge, and tears tend to propogate in polyimide film. All surfaces of a forming tool that come into contact with the flex circuit should be smooth, and free from any burrs or sharp edges.
Is there a CAD program to design flex circuit; such as Capture and Layout by Candence?
Unfortunately, no. Most flex designers will use a standard PCB routing program for the traces, and some will use a solid modeler such as Pro Engineer Sheet Metal module to simulate the mechanical features. This will get you close, but it really pays to have the design reviewed by a manufacturer or a design service specializing in flex circuit design. A few suggestions by an expert early on can save a LOT of headaches later.
Where can I get IPC 223?
You can get IPC-2223 from www.ipc.org. Under knowledge, click on standards, then on standards documents, then on purchase documents, then on design, then on 2223A.
Do you know what the projection is for the usage of flex circuit over the next 10 yrs and how does that compare to the last 10 years?
The flex circuit industry has been growing steadily for close to 2 decades, and has been widely accepted as a reliable interconnect method for over 10 years. Our flex circuit division has grown at over 20% per year for the last 5 years, and grew at 10%+ for many years prior to that. The growth in the military sector has driven a lot of the increase, but we are also seeing a surge in many commercial applications as well.
What is the finest line width/pitch of the current flex circuit?
Al lot depends upon the copper weight. On 1 ounce copper, we can go down to .004" lines and spaces with decent yields, but we would prefer .005" lines and spaces. With 1/2 ounce copper, we can go lower at reduced yields. The lowest we have gone is .0013" lines and spaces on 9 micron copper. But even with thin copper, that pitch caused very poor etch yields.
What CAD applications and file formats do you support when creating the flex circuits?
The most common file format for transferring PCB data is Gerber. We can also use DXF, and MAX (OrCAD) files. We can also convert many other file formats using translation software. Is there a particular software package you had in mind?
I believe that one of your slides indicated that the multiplier for bend radius to flex thickness changes as the bend angle increases beyond 90°. What are the multipliers?
It would depend on the circuit thickness and construction, and how far past 90 degrees you will be bending the flex. Unfortunately, there are no hard fast rules for determining safe bend radii beyond 90 degrees. For 135 degree bends, I would recommend at least 1.5X - 2X the minimums shown for 90 degrees bends. For 180 degree bends, 2X to 3X or more would be advisable.
What would the typical turn around time be for a "moderately" complex flex circuit to be manufactured?
That's a loaded question! There are quick turn shops that will turn a moderately complex flex in 1 week or less. They will be VERY expensive, and they will typically only run small prototype quantities. Many production vendors offer reduced leadtimes for prototypes, but their reduced leadtimes are all over the map. Minco's prototype group will usually produce circuits in 1 to 3 weeks depending upon complexity. "Moderate" complexity circuits would most likely fall in the 2 week time frame.
What size is a panel that a circuit is created on?
Panel size varies by manufacturer. The most common is 12" X 24" and 18" X 24". Most raw material comes in sheets that are 24" X 36" so that would probably be the largest panel you can go with. For most manufacturers, the plating line (and corresponding plating racks) will dictate the panel sizes they can run. Flex circuits that do not fit on an standard panel can be processed, but there will be a pretty substantial premium charge attached to it. Keep in mind, that on any panel size, not all of the panel is useable space. You typically lose approx .75" around the perimeter of the panels for plating rack clamps and tooling locations.
Do you know where there is information on heat bending a flex circuit?
The best source for that info is a flex manufacturer. There are a lot of variables that have to be taken into account when using heat to bend a flex circuit. First and foremost, NEVER use a heat gun to supply the heat!! The heat output from a heat gun can be 275 degrees 6 inches from the nozzle (no problem), and 800 degrees an inch or so closer. The latter temp will destroy the flex circuit. You should use an oven to preheat your circuits and then remove them one by one and quickly form them before they can cool. If you would like to discuss your application, please feel free to email or call me.
What guideline do you recommend for flex trace current capability? IPC internal layer is too high and external layer is too low.
The best idea is to refer to a current/conductor nomograph. There are many available online including one in the Minco design guide ( www.minco.com ). Also, if you think that current may be a problem, try to place the highest current conductors on the outer layers. The outer layers are generally panel plated resulting in close to, or over, an additional ounce of copper on those layers. Also, the outer layers are better able to dissipate heat than inner conductors. I hope this answers your question.