Using THT pads as SMD - Page 1
Forum section: electronics / projects

author: stelios
author_rank: Newbie
timestamp: December 08, 2014, 10:55:23 am
content: Hello,I'm working on a project that involves 4 different PCB sizes with the exact same circuit on them. (20x20mm, 20x45mm, 25x30mm, 40x40mm)The plan until now was to design and manufacture 4 different PCBs but apparently we have to perform EMC testing (including fcc part 15) for each design separately which is going to cost way to much for the relatively low volumes we are aiming. (~2k each design)There are 8 THT 0.1" pins, two at each corner, that connect to another PCB stacked on top of the same size as above. To shave some certification/testing cost I was thinking of designing just one PCB (20x20mm) and then design simple breakout boards for the rest. The breakout boards will have the 0.1" headers at the corners to connect to the top stacked PCB and 8x SMD pads for the smaller board to be soldered on.So, my question is, will the THT pad reflow on the SMD pad reliably, or will the solder just go through the hole and leave a poor joint between the two boards?I hope my explanation makes sense, feel free to propose an alternative but have in mind: the fewer exposed pads on the bottom of the whole assembly the better, has to be as labour free as possible, has to be cheap.

author: envisionelec
author_rank: Frequent Contributor
timestamp: December 08, 2014, 01:47:16 pm
content: What is the soldering process? Pin in paste?If it's wave soldering, you MAY be OK as the solder will continue to flow until limited by surface tension. I would run some trials on different geometries. If it's paste, the solder amount will be limited to the area within the stencil and the solder will be insufficient due to capillary action.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: December 08, 2014, 02:09:58 pm
content: EMC is a system level test, not a board level test. There isn't much you can do with an individual board, aside from functional testing and precompliance as far as the board itself goes.An example, off the shelf power supplies come with EMC ratings, but that doesn't mean they are guaranteed blameless in a system. There might be some unlucky combination of proximity and resonance which the power supply's noise couples into, who knows.Also, whose idea was it to make four sizes of the same circuit? I'm trying to imagine reasons what good that might be, but I don't get it.Tim

author: stelios
author_rank: Newbie
timestamp: December 08, 2014, 03:24:49 pm
content: Quote from: envisionelec on December 08, 2014, 01:47:16 pmIt's paste, the breakout board will have it's stencil and then the 20x20mm module will be picked and placed on it and then go through the reflow oven.So capillary action will suck most of the solder up into the hole.. I see.. Since the pad is on the edge though, maybe extending the SMD pad away from the board will help?Quote from: T3sl4co1l on December 08, 2014, 02:09:58 pmWell, I didn't mention that each board is a BLE module so, being a transmitter, we have to certify each design separately; they won't accept all sizes as a single module under one FCC ID. There is the option of 'worst in family' but then again, it costs more overall. For EMC yes, you are right, the system will be tested as a whole, all modules with all the different versions of boards that go on top. (see below)As I mentioned, another board is stacked on the top of this one and there are various versions of it. The size is defined from the components that go on this top board and since there are more than 15 top versions and size is critical, 4 bottom boards is not that bad.

author: mikeselectricstuff
author_rank: Super Contributor
timestamp: December 08, 2014, 08:29:46 pm
content: Quote from: stelios on December 08, 2014, 03:24:49 pmWhy do you think splitting into multiple boards will help - approval is on the product, not boards within it.

author: sca
author_rank: Regular Contributor
timestamp: December 08, 2014, 08:44:13 pm
content: If you do go board-on-board, look at how some of the small bluetooth or RF modules (eg the Hope modules) do it, with what look like sectioned TH pads plated on the edge.sca

author: IconicPCB
author_rank: Super Contributor
timestamp: December 08, 2014, 09:42:16 pm
content: SCA gives you good advice... those sectioned holes are known as castellations

author: stelios
author_rank: Newbie
timestamp: December 09, 2014, 12:53:44 am
content: Quote from: mikeselectricstuff on December 08, 2014, 08:29:46 pmBy splitting into multiple boards we can have all the BLE related stuff including a shielding can on one board and certify that as a radio module. (radio tests, part15, BT sig qualifications etc) The final assembled modules would be tested only for EMC in groups of 5 or even all 15 at the same time.If we don't do this, then each one of the 15 modules would have to be tested for everything including lengthy radio tests that cost a lot. A couple of test houses seem to agree on this and is reflected on their quotes.Quote from: sca on December 08, 2014, 08:44:13 pmQuote from: IconicPCB on December 08, 2014, 09:42:16 pmActually, I am considering those 'half moon' pads... problem is, I need the holes to mount the 0.1" headers on the 20x20mm version. I will probably end up using both.I'm quite new to DFM and all the regulations so thanks a lot for your input everyone!

author: RobertHolcombe
author_rank: Frequent Contributor
timestamp: December 09, 2014, 05:30:28 am
content: I removed the can from a GPS module on some write-off equipment for reference, see attached photo