Thin FR4 PCB's and high current - Page 1
Forum section: electronics / general

author: OwenH
author_rank: Contributor
timestamp: December 07, 2022, 08:31:16 pm
content: Hi,In the past I've only used 1.6mm thick PCB's but I now need more clearance in a tight enclosure. I'm thinking of using a board that's 0.5mm thick FR4 but I've never worked with anything this thin in the past. This design will need to handle 12 amps so there will be wide 2oz copper and 4 layers. The board will be taped with a thermal adhesive tape to an aluminium enclosure so I don't think the rigidity of the board should be an issue. I don't see why high current with a thin pub should be any worse with a thin pcb? Maybe its even better? How flexible is a thin pcb and is this an issue for staying flat while reflowing?Thanks,Owen

author: mikeselectricstuff
author_rank: Super Contributor
timestamp: December 07, 2022, 08:41:26 pm
content: on 0.5mm, flatness/warping when reflowing may well be an issue. I'd stick to 0.8 if you can, if only because it's more common so you have more choice of suppliers. 0.5mm with heavy copper is a bit of an oddball combination.Something you should try to do is balance the amount of copper on each layer, as unbalanced copper can cause warping.I had this really bad on a 2-layer 0.2mm thick board, not sure how bad it would be on 0.5m but balancing can't hurt.I'd also avoid solid pours, and use a mesh pour instead - tweaking the line width of the mesh is a useful way to balance the copper between layers.

author: OwenH
author_rank: Contributor
timestamp: December 07, 2022, 10:08:52 pm
content: Hopefully my copper is relatively well balanced since I'm maximising the planes for thermal disapation however I will have another look. Do you think that because this board will be only 25mm x 35mm that will help reduce any warping? Also what's your feeling on high current in a thin PCB?Thanks!

author: T3sl4co1l
author_rank: Super Contributor
timestamp: December 07, 2022, 11:37:11 pm
content: I wouldn't worry about warpage on something that size, and as long as copper will be fairly even.They might not do 0.5mm total with 4 x 2oz anyway, you need enough resin in the prepreg to fill in gaps between traces -- but 0.8 and 1mm should be fine (and reasonably priced still).12A isn't exactly high current, you might even be able to do it in 2 layers, unless that's driven more by complexity or component density. I would just worry if you're trying to get more than a couple watts out of that. Which, probably with high-K thermal pads on both sides, clamping it between heatsinks, something that size could do 10W or so, maybe even a bit more, but that's also a fairly demanding assembly. Say 3-5W with a single side thermal pad would probably be more practical.Tim

author: jayx
author_rank: Regular Contributor
timestamp: December 09, 2022, 02:09:42 pm
content: 0.5mm is very thin and will be very flexible. Also at this size you'll need them on a panel. Most likely it will be problematic for paste print, P&P and reflow, unless you add some stiffener.

author: OwenH
author_rank: Contributor
timestamp: December 29, 2022, 09:40:45 am
content: Do you think this would still be an issue with 0.8mm? I am going to have them panellised but I don't have any space for a stiffener.

author: Psi
author_rank: Super Contributor
timestamp: December 29, 2022, 09:43:07 am
content: yeah, agreed. Stay away from anything less than 0.8mm

author: loki42
author_rank: Frequent Contributor
timestamp: December 29, 2022, 12:21:24 pm
content: I've used thin (0.6 and 0.boards because I need to for touch stuff. Avoid them if you can. I get some that are so warped the stencil printer complains. You'll need good board support and might need things like vacuum fixtures. It'll make placement harder. 1.2 and you've got no problems with washing sagging and they print easily.

author: Yellofriend
author_rank: Regular Contributor
timestamp: December 29, 2022, 01:27:33 pm
content: You want a PCB thickness of 0.5mm, 2oz copper is already 70µm = 0.07mm - so you have already 0.28mm copper thickness. Prepreg is typically 0.2mm (one layer 7628), but there is thinner too.Quite a challenge.

author: OwenH
author_rank: Contributor
timestamp: December 29, 2022, 01:43:12 pm
content: Well I have decided that I won't go with 0.5 as it sounds like trouble. I guess I would just like to know if 0.8 is likely to be ok? the panel size is about 80 x 110mm. The first set of boards would be assembled by hand and I don't really want to deal with a warped panel in the reflow oven. I think my copper is well balanced so hopefully that will help.

author: Yellofriend
author_rank: Regular Contributor
timestamp: December 29, 2022, 02:22:35 pm
content: Quote from: OwenH on December 29, 2022, 01:43:12 pmTry 2x 7628 70/70 laminate (~0.2mm + 0.07 x 2 = 0.34) and one 7628 in the middle = 0.88mm (or a bit more with ~25µm plating, solder mask.. )I don't see any warpage issue.