PCB X-Ray Services - Page 1
Forum section: electronics / general

author: 48X24X48X
author_rank: Frequent Contributor
timestamp: March 29, 2017, 06:08:51 am
content: Hi guys,Does anyone know any company that would provide PCB x-ray services? Like they take an X-ray image of your assembled board and send a soft copy to you.I have been prototyping a board with an LGA module package for 3 revision now and it always end up with shorts. I'm on the brink of giving up on the design but I could give it another try providing I know how the joints under the module would look like.And what are the cost like?

author: voltsandjolts
author_rank: Unknown
timestamp: March 29, 2017, 08:37:21 am
content: Maybe mike can help if you ask nicely.mikeselectricstuffhttps://www.eevblog.com/forum/profile/?u=906

author: Kean
author_rank: Unknown
timestamp: March 29, 2017, 09:04:31 am
content: Or your local dentist?

author: 48X24X48X
author_rank: Frequent Contributor
timestamp: March 30, 2017, 07:11:37 am
content: Hi guys,Thanks for the suggestions! Especially on mike!After discussing the issue with the module distributor, it proved that I'm not alone, at least I'm not crazy.Most of the customers have about 30% rate of short circuit with the module. That is scary enough to go into production.Throw in the towel I guess unfortunately. By the way, it's a Sierra Wireless 3G module.

author: Leolabs
author_rank: Newbie
timestamp: March 30, 2017, 09:05:57 am
content: Why dont u try to check with Vitrox for their AXI machine?Since u are from Malaysia.....

author: wraper
author_rank: Unknown
timestamp: March 30, 2017, 09:38:25 am
content: Sounds like a wrong amount of solder paste applied, very important for LGA. I hope you don't fill all of the area in the center with a solder paste. Sounds like issues with solder paste stencils you used rather than PCB revisions.

author: Kean
author_rank: Unknown
timestamp: March 30, 2017, 09:51:26 am
content: I agree with @wraper - too much paste. I've seen the same with LCC GPS modules that I've prototyped.You may need to use smaller apertures on the stencil for these pads, or if that isn't possible you may need to use a thinner stencil material.

author: mikeselectricstuff
author_rank: Super Contributor
timestamp: March 30, 2017, 10:14:15 am
content: Most big assembly houses will have x-ray facilities - they may do it as a service if they're set up for it, or maybe do a freebie if you ask nicely ( or suggest they will get the job for production)!If this is something for production you should talk to potential assembly houses as they should be able to advise on assembly issues like this.Dental x-rays probably don't have the penetration or resolution - for BGA type stuff you really need something designed for the job.

author: voltsandjolts
author_rank: Unknown
timestamp: March 30, 2017, 10:33:52 am
content: Here's a QFP on PCB courtesy of our local dentist showing vias, tracks and bonds.

author: 48X24X48X
author_rank: Frequent Contributor
timestamp: March 30, 2017, 12:41:59 pm
content: Yes, the very first thing I thought was the stencil opening. So, I made them smaller in 2nd revision and even smaller in 3rd revision. Even their recommended stencil opening shape was a weird 3 trapezoid shape for a single square pad. Here's an example of how the pads looks like without the module mounted.QuoteI made them in small quantity that renders sub contracting them to local factory not feasible.QuoteI didn't know there's even an option here. Let me check them out. Thanks.

author: wraper
author_rank: Unknown
timestamp: March 30, 2017, 04:29:45 pm
content: WTF is with that central pad. It should be a solid copper polygon without solder mask.

author: mikeselectricstuff
author_rank: Super Contributor
timestamp: March 30, 2017, 04:34:53 pm
content: Quote from: wraper on March 30, 2017, 04:29:45 pmNot necessarily - that can put too much paste on. Could be it's a BGA/LGA-style package with multiple pads rather than a single pad.

author: james_s
author_rank: Super Contributor
timestamp: March 30, 2017, 05:14:48 pm
content: You might try asking a vet (animal doctor), they usually have xray machines and if they aren't busy one may be willing to xray a board for you. The machine I've dealt with is essentially the same as those made for human medical diagnosis with a few changes to make it easier to use it on animals which tend to not hold still on request.

author: wraper
author_rank: Unknown
timestamp: March 30, 2017, 06:38:22 pm
content: Quote from: mikeselectricstuff on March 30, 2017, 04:34:53 pmHow is this related? You should never do 100% paste coverage of the pad. In some cases some solder mask may be left on the pad, to divide it in areas but no such nonsense with separate pads. Each with thermal relief, defeating the main purpose of the package.

author: wraper
author_rank: Unknown
timestamp: March 30, 2017, 06:42:51 pm
content: Same pad size, different amount of solder per same area. Solder will be squeezed out from the pads with higher amount of solder.

author: wraper
author_rank: Unknown
timestamp: March 31, 2017, 02:51:11 am
content: I think I know which part he is using. I see the recommended footprint there with separate pads on the center (thermal reliefs are still a nonsense). No solder paste stencil recommendation though.http://www.eltech.spb.ru/item/HL8548%20&%20HL8548-G.pdfEDIT, looks like significantly less solder on their photo. What is the stencil thickness, is 100% aperture used?

author: amitchell
author_rank: Regular Contributor
timestamp: March 31, 2017, 03:34:53 am
content: It also looks like the solders being used are wetting the pads differently, and have a different surface finish. I have heard that the LGA packages are fussy, maybe figure out what solder the chips manufacturer recommends. Play with the reflow curve too.Rounding the corners of the pads may also help prevent bridging.

author: 48X24X48X
author_rank: Frequent Contributor
timestamp: March 31, 2017, 05:57:58 am
content: QuoteI was about to post that red PCB picture. The different is my GND pads has thermal relief, theirs no. This could be the potential problem. Attached is my stencil opening. For the outer small pads, I followed their recommended stencil opening size but for the inner GND pads I weren't able to generate the complex pattern they recommended (will need to some primitive drawing in KiCad to get it properly done), so I reduced them quite a bit especially on the most outer GND pads. They recommend 0.125 mm stencil thickness and I used 0.12 mm which is even thinner, by a bit.QuoteI used ENIG for all my PCB and that is the recommended surface finish. In terms of reflow curve, I followed a generic lead-free curve.QuoteBoth of that 2 pads have same stencil opening size.But, here's the catch. There is one and only one net that shorts with GND. Which is the main power supply to the module. See the attached image. It's weird because if we assume that the solder paste does flow from the center pad (solder mask is there though to prevent this) to the outer pads, why only this particular NET. It would have shorts with other pins too. I have checked those pins that were broken out and use in the design and there's no short to GND.

author: amitchell
author_rank: Regular Contributor
timestamp: March 31, 2017, 06:23:01 am
content: QuoteSorry for not being clear, I meant the solder had a different surface "finish" i.e., it looks like a different type of solder. Maybe an avenue to explore, along with the reflow curve.

author: KE5FX
author_rank: Super Contributor
timestamp: March 31, 2017, 06:38:10 am
content: The central pad looks fine to me. The best practice is probably lots of blind vias under a continuous pad, but board houses charge big-time for those, and most ICs don't care that much.If you've already shrunk your paste apertures, I'd try a thinner stencil.

author: SMdude
author_rank: Frequent Contributor
timestamp: March 31, 2017, 07:12:05 am
content: Are you able to remove one and confirm that the module hasn't actually shorted within the module itself?To me it seems odd that that is the only net experiencing shorts. The solder balls don't look excessive to me.Are you using a low temp paste?Have you tried removing the paste from a few of the pads nearest the V_batt pads?Or a short somewhere else, though I assume you have eliminated everything to end up at the module..Cheers

author: voltsandjolts
author_rank: Unknown
timestamp: March 31, 2017, 07:27:02 am
content: The gap between the centre GND pads and the left/right pads looks smaller than the gap to the top/bottom pads.So why are the shorts not happening at the sides?As SMdude says, is the short in the module?

author: Spikee
author_rank: Frequent Contributor
timestamp: March 31, 2017, 09:52:49 am
content: Quote from: voltsandjolts on March 31, 2017, 07:27:02 amI have had a 50% failure rate for a RF module which was configured on the 433MHz band , the 915 and 868 MHz modules had no issue at all.The shorts were within the modules itself not on the board I designed. I have seen this multiple times.I always do 100% past coverage other than for BGA's *which are a pain in the ass" , 0.1mm thick stainless steel stencil or smaller.I have heard about 0.07mm thick stencils before at assembly houses.

author: 48X24X48X
author_rank: Frequent Contributor
timestamp: March 31, 2017, 10:53:09 am
content: QuoteTheir reflow curve recommended on their datasheet looks pretty standard. But, they didn't mention anything on the paste itself other than it should be lead-free.QuoteI have tried in the past, by the time I have the amount of heat applied on the board, the FR4 started to melt. The center pad is holding the module to the PCB too strongly I guess. No, I'm not using a low temperature paste. I did tried once removing he outer most ring GND paste and the module does work although not reliably. But, in the sense it doesn't short. This was in the 1st revision of my prototype where the paste opening was 95% of the pad size. In the case where other parts of the circuitry could have shorts, I have also removed them totally before.QuoteI think a 0.1mm thickness would worth the try.QuoteThe module is not symmetrical, more like a rectangle. That's the odd part, why not short the pins to the left and right?There are 2 variation of the module actually, one the 3G version and the other is 2G version. They are pin compatible. But, the 2G module body is what you would expect from a module (metal can on top, and visible FR4 PCB at the bottom). However, the 3G version doesn't look metal at all (it's some coating I guess). If you do a continuity test around the 3G module except the bottom part without pads, it's all connected to GND even on the sharp edges. The individual non-GND pads obviously is not shorted to GND.But a 30% mount issue as reported by my distributor really say something is not right somewhere...And it ain't cheap a piece.QuoteRN2483 I guess? I think a 0.10mm looks like a possible solution.As for now, I'm moving on and ditching the Sierra Wireless series. Even if I managed to figure it out, I believe the yield could still be less than optimal. But, I will reorder a thinner stencil with even smaller aperture to try.

author: wraper
author_rank: Unknown
timestamp: March 31, 2017, 11:26:34 am
content: Quote from: 48X24X48X on March 31, 2017, 05:57:58 amAsking for trouble, when there is a different thickness of solder in different places, thicker solder will be squeezed out.QuoteHave you measured resistance on the new not soldered module? Is it even soldering issue at all? You said you always get shorts, does it mean in 100% cases?I don't know anything about these modules, and not into RF but if depletion-mode GaAs MOSFETs are used, it will be dead short on the power before voltage is applied to their gates. So dunno, maybe your module does not start properly.QuoteYou need a preheating on the bottom for this.

author: mikeselectricstuff
author_rank: Super Contributor
timestamp: March 31, 2017, 11:50:22 am
content: The idea of re-reflowing an RF module seems a bit dubious to me, with the combination of high cost and delicate RF stuff. I'd be inclined to go for something that plugs in, which would be much lower risk.

author: Spikee
author_rank: Frequent Contributor
timestamp: March 31, 2017, 12:02:20 pm
content: I think it mostly has to do with bad pcb design (practices) on their end. Which causes (extra) thumb-stoning / excess solder to mess up things whenever it is re-flowed afterwards.My solution let the assembly house do the soldering by hand (pads were accessible) -> no more excessive failures.It was a different RF module than the one you mentioned.Just finished design on a board that has the SIMCOM SIM868 in LGA package, no short / assembly issues so far on that part.

author: wraper
author_rank: Unknown
timestamp: March 31, 2017, 12:30:47 pm
content: Quote from: Spikee on March 31, 2017, 12:02:20 pmTombstoning is not possible when reheating the parts which are already reflowed.