Flexible PCBs - looking for advice - Page 1
Forum section: electronics / general

author: ebastler
author_rank: Super Contributor
timestamp: April 04, 2022, 06:45:13 am
content: I am considering designing my first flexible PCB: A heater foil plus some sensors (temperature and others), approx. 30 * 25 mm² in size. While I have designed rigid 2- and 4-layer PCBs before, flexible material is a first for me. I have looked at PCBWay's ordering page, but still have many questions... I trust that some of you have done this before and appreciate your help!I could not find dedicated design rules for flex PCBs on the PCBWay page. For PCBWay or other manufacturers, do the same rules apply as for rigid PCBs, e.g. regarding minimum widths and distances, drill sizes etc.?  Ah, minimum trace widths and hole sizes are actually specified right on the order page. How about contour milling and slots? Are they milled at all or laser cut? Does that imply narrower cuts are possible?Operating temperature would be around 70 °C. That should not be a problem for the PCB material as well as the adhesive, since they have to withstand reflow soldering too, right?Any specific advice on designing the heater traces?  Besides the standard 35 µm copper thickness I notice an option for 18 µm (0.5 oz/ft²), half of the standard with rigid PCBs. For long-term stability, would it be preferrable to opt for thicker copper and narrower traces (6 mil), or 18 µm copper with wider traces (10 mil)? The modest 70°C operating temperature applies again.PCBWay lets me opt for an adhesive layer (to attach the PCB to a support). Will this always be applied to the "bottom" of the PCB? Can I specify where the adhesive goes, with a dedicated layer in the Gerber files?Would you recommend PCBWay as a manufacturer or do you have other preferences?Many thanks for your advice!

author: mon2
author_rank: Frequent Contributor
timestamp: April 04, 2022, 04:38:50 pm
content: Sharing a contact for a reliable flex pcb shop in Asia. They do work for the large fruit company.

author: Day101
author_rank: Contributor
timestamp: April 06, 2022, 06:27:34 am
content: The track width and spacing is better on flex than standard pcbs.I think your sales rep should be able to answer your questions but I would ask them one at a timeyou are probably better to prototype on rigid PCBs before ordering flex to minimise cost.The flex material is very heat resistant and very durable. 70C will not be a problem.I would order a test PCB with a range of track thicknesses and spacings to see what heats best. You can probably include solder bridges to make the coils longer or shorter. If you're careful. 4 layer standard PCBs are almost the same cost as 2 layer.4 layer flex is much more expensive. The cost drop is massive when you order hundreds or thousands of flex but small batches are expensive

author: ebastler
author_rank: Super Contributor
timestamp: April 06, 2022, 11:41:47 am
content: Thank you both for the feedback!I can confirm the the minimum track width & spacing is surprisingly small for PCBWay's flex boards, just 60 µm (2.5 mil)! That gives me some confidence that they will also etch my heater lines to pretty close width tolerances.Drill sizes need to be larger than for standard PCBs, with a mimimum of 0.35 mm.Edit: I misread that; minimum hole size is 0.15 mm, mimimum via pad size 0.35 mm.Tech support at PCBWay has been very responsive and has confirmed that they can produce narrower unplated slots in the flex PCBs, with a 0.2 mm mimimum slot width. (Their minimum for FR4 PCBs is 0.8 mm.)I expect to submit a draft PCB for their review tonight, and hopefully to production tomorrow. I'll come back here later and comment on the learnings and results.

author: ebastler
author_rank: Super Contributor
timestamp: April 07, 2022, 10:42:54 am
content: First feedback on PCBWay, about their ordering process:While they provide an online quoting function, that is not very useful. It is a non-binding estimate, and they will give you the actual price after they have reviewed your Gerbers. And that price may deviate significantly from the online quote.In my case the online quote was $76 for the PCB itself, but the actual price was $120. I enquired about the reason for the big difference and they did not state any specific cost drivers. It's "take it or leave it".I did bite the bullet and place the order, but it was not a great first experience. Hoping that timely manufacturing and good quality can make up for that...