PCB or Solder stencil issue - Page 1
Forum section: electronics / general

author: GoatFounDeR
author_rank: Newbie
timestamp: May 26, 2016, 01:00:55 pm
content: Hi,We are having some failures on a run of our product. I've spend the last two day's trying to figure out what is going on, and it basically came down to the PCB factory blaming the oven profile used, and the Assembly house blaming the PCB and paste stencil tolerance.I'm in no way an expert in these kind of things, and I would like to get a 3rd party opinion from someone here, that obviously have no special interest in the outcome. Below follows the details and photos that I have collected thus far.I've asked the PCB manufacturer to measure a panel for me and make sure that the PCB is within tolerance.Maximum Deviation: 0.084Minimum Deviation: 0.002Average Deviation: 0.0445I have the full report if more info on this will help. It seems like the maximum and minimum tolerance that they will allow is 0.100 and -0.100To this degree the stencil was also checked and made sure that it was cut to specification. The Assembly house told me that they did do a 10% aperture reduction. But did not do the aperture reduction on the BGA. But they insist that the panel deviation is the cause of the failure of the boards. Because the paste does not align perfectly with the pad. Attached is photos showing the BGA. There are 3 boards per panel. 1 is left. 2 middle, and 3 right. Here is a link to the rest of the un-cropped iamges:https://www.dropbox.com/s/xue5y36oobx55uv/paste-photos.zip?dl=0Some details about the BGA:Pad Diameter: 0.3Ball Pitch: 0.8Ball Diameter: 0.3Ball Count: 324Symptom of the Failure is basically some board will not work, unless I press down on the BGA, and some features (like USB) not working. I have had quite a bit of success working with the repair technician in getting the boards working again by reflowing the CPU.We took x-rays of the BGA as well. The x-ray technician said that everything looked good and aligned to him. But yet the CPU does not seem to start without pressure. I read up a bit and it might be due to "brittle solder"? I'm still awaiting the x-ray images.So if anyone can shed some light on this for me. I would greatly appreciate it. Is the tolerance of the PCB to low? Does the amount of past look like it will be enough. Is the paste positioned correctly. Any other questions I need to ask the manufacturer's?

author: D3f1ant
author_rank: Frequent Contributor
timestamp: May 26, 2016, 08:12:15 pm
content: The prints look ok and if X-ray shows alignment is fine then I suspect your not getting enough heat into the bga. Assuming your peak board temperature is ok, try increasing the preheat soak time. If that doesn't help, increase the peak soak time. Your profile is probably very very close to correct but not quite there.

author: JuKu
author_rank: Frequent Contributor
timestamp: May 27, 2016, 07:19:13 am
content: > some board will not work, unless I press down on the BGASounds like a failed solder joints. I agree with D3f1ant, the profile might not be quite right. But first picture, upper right and second picture, low right look like shorts on the PCB?

author: 48X24X48X
author_rank: Frequent Contributor
timestamp: May 27, 2016, 07:24:12 am
content: If the BGA is the largest guy on the PCB, it's best the thermal profile to be measured on him instead on other parts of the board. This is what my factory assembler told if you have a bigger part like a module or a large BGA. Of course, the other component will suffer a little, but that is tolerable.

author: Psi
author_rank: Super Contributor
timestamp: May 27, 2016, 07:37:48 am
content: Not enough heat/time was my first thought as well.Can you pry off one of the BGAs with a screwdriver and inspect the balls at the center?If they didn't get fully soldered the center balls should break off pretty easy.It will ruin the board, but might be worth the sacrifice.

author: wraper
author_rank: Unknown
timestamp: May 27, 2016, 08:06:21 am
content: PCB/paste looks fine. I would certainly blame the assembly house. Might be something like thishttp://www.ko-ki.co.jp/en/memo/memo_06.html