Daughter PCB....make it like an SOIC8...(the legs) - Page 1
Forum section: electronics / general

author: Faringdon
author_rank: Super Contributor
timestamp: May 11, 2022, 10:18:04 am
content: If i may please ask....We are doing a 2kW SMPS. PFC then Half Bridge LLC.The main board has the chunky components on it and is rather large for this initial prototype (500mm x 400mm).Therefore, we dare not put SMD on this potentially ‘bendy’ board. Though we need the Gate drive components to be SMD for sake of reducing stray L. As such, do you think the attached method is good for doing the gate drive with SMD daughter boards?Its basically making the daughter board into a little kind of SMD component. (As SMD, The daughter board will be easier to remove if we need to, during testing.)BTW, the cheap Chinese PCB Fabs don’t seem to be doing >1.6mm thickness on the cheap rates for our main board size, so we must use 1.6mm thick main board.The below is the only thing I can find so far, but we need the legs to come “out” so we can solder them to the mains board SMD pads…..https://uk.farnell.com/wurth-elektronik/61000418221/header-2-54mm-pin-smt-4way/dp/2356209Is there such “SMD header” as in the attached picture?Basically we just want a header which will kind of put "legs" onto the daughter PCB.....to make it kind of like a SOIC8, type of thing...those kind of SMD "legs"I mean, the below could kind of be hacked to do it..https://uk.farnell.com/multicomp/mc34747/header-1-row-r-angle-3way/dp/1593427....but its going to be a bit "raised" compared to a SOIC, kind of thing.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: May 11, 2022, 10:40:57 am
content: Wowee! Last time I saw a board that big, it was because it was stuffed full of TTL chips. Don't think I even saw a power board that big, not at 2kW anyways, maybe close in the 10-20kW (induction heating) machines I've seen. Or there was one that was full panel size (like 24 x 36") but that was more like 30kW, ooold ass SCR based unit, tons of free space on the board anyway, clearly they didn't care.Granted, they were using even bulkier components than you, I almost guarantee... freaking unit was near 50kg, just the 10kW "tabletop" model even. But yeh, no choice but to chassis mount components like that.Tim

author: spostma
author_rank: Regular Contributor
timestamp: May 11, 2022, 02:11:16 pm
content: I would make a daugterboard with all SMD components,and solder it high above the motherboard with two straight pin headers,using the entire pin length as flexible interconnects.Place the two pin header not too far apart.

author: Faringdon
author_rank: Super Contributor
timestamp: May 11, 2022, 03:36:57 pm
content: Thanks, so may i enquire.....you wouldnt use these..?i could use this on the sub board.......https://uk.farnell.com/multicomp/2213r-06g/header-2-row-r-angle-6way/dp/1593449?st=2213r-06g...and then this socket on the main board......https://uk.farnell.com/harwin/m20-7830446/socket-pcb-0-1-4-4way/dp/7991991.....because soldering PTH header pins in is going to be difficult if we have to remove it....though if you say soldering is essential due to the 250khz and high tr (<100ns), then we will solder it?

author: 3roomlab
author_rank: Frequent Contributor
timestamp: May 11, 2022, 04:39:13 pm
content: you are talking about something like this?

author: julian1
author_rank: Frequent Contributor
timestamp: May 11, 2022, 10:45:15 pm
content: Are the series inductors L9/L10 a measure to suppress gate ringing?

author: Faringdon
author_rank: Super Contributor
timestamp: May 11, 2022, 11:00:20 pm
content: Thanks, the series inductors are what i put in to imitate leakage inductance.3roomlab...that looks fantastic!!...thanks for the tip...it looks like a great way to do this...youve made basically an 8 pin component...and nothing atop that header will suffer bending stresss....and can be SMD no problem......even quite easy to desolder from the main board if necessary.

author: Faringdon
author_rank: Super Contributor
timestamp: May 12, 2022, 10:28:44 pm
content: Hi,So, as we all know, if you have an SMPS on a big main board, which comprises heavy components, then you cant put SMD on it, as it will crack or go open when the main board flexes…so , for prototyping only, what about the attached method to get SMD on to a big main SMPS board?Its just a little PCB, with SMD components on it…..and it has thru holes which line up with holes on the main PCB, and little bits of tinned copper wire are pushed through these holes to solder the little SMD PCB to the main board.....there aren't even any "sticking out legs", so lead inductances will be to a minimum, which is good for things like gate drive componentry and current sense circuitry etc.Do you see any problems with this?

author: T3sl4co1l
author_rank: Super Contributor
timestamp: May 13, 2022, 06:09:47 am
content: So anyway, what's wrong with headers? Everyone else uses headers, they're fine.I even did one some years ago, it's... well, it doesn't show very well since everything's the same flat shading, but...https://www.seventransistorlabs.com/Images/PSPWMFC_Layout.pngnotice near the bottom-right one of these,https://www.seventransistorlabs.com/Images/PSPWM_PCB2.jpgon 0.1" headers and snap-in standoffs, nothing could be simpler. Can be soldered in if needed, can fit other components under it, no problem. Or use right-angle headers for vertical mount and save layout area, very commonly seen in commercial builds to pack in extra circuitry in a constrained design.Tim

author: Faringdon
author_rank: Super Contributor
timestamp: May 13, 2022, 09:26:52 am
content: Thanks, do you think cheap 0.1" header into socket woudl be ok for connecting up 400Vdc?......(sub 100mA of current).....AYK, not hot-plugging.....those cheap 0.1" socket housings look terribly flimsy inside.

author: jpanhalt
author_rank: Super Contributor
timestamp: May 13, 2022, 09:45:35 am
content: If you are referring to a female mate, I do not like the simple/cheap ones where the contact looks like just 2 tines on a fork. The contact is loose and their insertion cycle life is low. I like Millmax (like the circular ones shown) and anything with a cantilever type of contact (think "Berg" type of contact). Samtec and Amp make them, probably others.Why not individual, soldered copper wires? Easy to remove and redo. My old Tek scope uses soldered board-2-board headers, but they typical square-section brass. Those are darder to separate.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: May 13, 2022, 01:27:47 pm
content: 0.1" headers are fine at voltage, just skip over a few pins, or remove the inbetween pins entirely (or use 0.2" or whatever). There's also (individually) shrouded families from Samtec and others, rated for high voltages (600V?) between positions (not that you'll be able to use such voltages on the PCB, hah; well, maybe with some creepage slots between pins?).Quote from: jpanhalt on May 13, 2022, 09:45:35 amStrange, I find the punched-sheet box kind to be rather flimsy, vs. the fork style (e.g. Samtec, uh, "Tiger Claw" do they call it?) being tighter, more secure. Though I can't comment on insertion life, beyond that it's been more than enough for my dev/proto purposes.Tim

author: eugene
author_rank: Frequent Contributor
timestamp: May 13, 2022, 02:58:46 pm
content: Quote from: evb149 on May 13, 2022, 03:59:17 amI expect any prototype I make to come back broken after being handled by any other engineer. The first thing I do when someone hands me one of theirs is break it. Not always on purpose!

author: Faringdon
author_rank: Super Contributor
timestamp: May 15, 2022, 09:48:53 am
content: Thanks T3sl4co1l for your post #9 above....thats a great SMD platform above the main board!.....you have a single SIL connector, on one side, then advantageously you have what looks like 2 plastic spacers on the other...which does a great job of mechanically decoupling it from bending stress on the main board, whilst still holding it well.Sorry for the stupid question, but how did you manage to get the plastic spacers which advantageously match the height of the SIL connector?

author: T3sl4co1l
author_rank: Super Contributor
timestamp: May 15, 2022, 11:20:04 am
content: Not exactly hard to find...https://www.digikey.com/en/products/detail/keystone-electronics/8882/2345366These are the snap-in (friction lock) kind, good enough for prototyping, maybe not ideal for production builds. There are others with snaps or whatever, or just use screw mount hardware to do it.Seem that there are standard spacer heights a little bit above and below the exact seating height of typical e.g. Berg strip, which is kind of annoying, but having it higher is better than lower. There's plenty of contact engagement to allow for an extra say 1mm of height there.For vertical boards, consider card guides e.g.https://www.digikey.com/en/products/detail/bivar-inc/VG2-2/3102495make it a bit wider than the header, then place these on either side for secure alignment.Some kind of latch holder to keep it retained in place is a good idea for production purposes. Or add tabs / screw holes and use mounting brackets, or short-pin header and directly solder the thing (as usually seen in commercial PSUs).Can also use multiple headers top and bottom, so that there's a wider footprint where it plugs in. That is, instead of one single-row header, split it into 3+ lengths, on top/bottom/top sides say. Or use 2-row headers to a similar purpose (which can even be SMT soldered directly over the board edge).Tim

author: Faringdon
author_rank: Super Contributor
timestamp: May 15, 2022, 03:50:50 pm
content: Thanks...So, We are doing 2Kw LLC Half bridge with vin = 400vdc.The hi side drives are (on the prototype) being done as in the attached. The little sub-boards allow us to use SMD components without risk of them suffering possible flexion stress, which they would get if they were on the big main board. (the header will be soldered to the main board)Do you think this single SIL connection will stop flexion on the main board from occurring to the sub board?(also attached is an LTspice of this gate drive)