Shorted capacitors from assembly house? - Page 1
Forum section: electronics / projects

author: Pack34
author_rank: Frequent Contributor
timestamp: September 30, 2016, 05:32:50 pm
content: I've been having an odd recurring issue when getting some boards made by an assembly house where a random capacitor is shorted. This seems to be happening on about 20% of the boards recieved.Since the short is there before the board has been powered, is it possible to have a thermal failure caused by the reflow oven being too hot?I'm leaning towards the oven being too hot due to some wire-wound inductors being discolored and I'm unable to find evidence of solder balling.Is this possible?

author: jitter
author_rank: Frequent Contributor
timestamp: September 30, 2016, 05:46:53 pm
content: "Recurring" and "20% rejects" should be ringing alarm bells. Have you contacted your assembly house?I guess overtemperature could be a possibility, but so could a bad batch of capacitors be. A yellowish kind of tarnishing (oxidation like) on the leadfree finish of e.g. component leads after soldering is something I regularly see, and is not an indicator of overtemperature.Do you let the assembly house also do the purchasing of the components?Edit: are these caps by any chance tantalum capacitors?

author: Pack34
author_rank: Frequent Contributor
timestamp: September 30, 2016, 06:07:24 pm
content: Quote from: jitter on September 30, 2016, 05:46:53 pmThe production manager has scheduled a conference call. This latest batch is unheard of at 20%. Usually we'll get one or two in our small batches with a bad capacitor but this one is unreal.The assembly house does the component purchasing.They're ceramics X5R capacitors and are supposed to be these:http://www.digikey.com/product-detail/en/taiyo-yuden/TMK316BJ106KL-T/587-1337-1-ND/931114

author: free_electron
author_rank: Super Contributor
timestamp: September 30, 2016, 06:35:47 pm
content: improper temperature control. not dry-packed . could be mulitple possiblities.but . it could also be board design ! how close are these parts ot the edge. is the board mechanicalle depanelised ( v-score ) are they subject to flexing after assembly.could also be usage of wrong flux remover ...

author: Pack34
author_rank: Frequent Contributor
timestamp: September 30, 2016, 08:23:03 pm
content: Quote from: free_electron on September 30, 2016, 06:35:47 pmI didn't think of that. The edges of the pads are 25 mils from the edge of the board. Looking at this from the right angle I think I saw some film across the capacitor leads. So I took some water and a brush to it and then heated the pads with an iron. It looks like the short went away.It's a 1206 part, parallel to the board edge and spaced about 25 mils in, it is also parallel to a SOT23-6 package with only a 15 mil spacing between the two. I think this may be the answer and was able to get the short to go away on the ones that were not powered on.

author: Ice-Tea
author_rank: Super Contributor
timestamp: September 30, 2016, 08:32:46 pm
content: 25 mils from the edge.. of what? A V-score? In that case, expect more troubles...

author: AndyC_772
author_rank: Super Contributor
timestamp: September 30, 2016, 09:05:35 pm
content: Could be a fracture caused by mechanical stress. It doesn't take much to cause a hairline crack, and if the adjacent conductive layers touch then there's your short.PCB bending when the panel is split up is one possibility. Another is that the component's height is programmed wrong in the pick & place machine, and it tries to smack the component through the PCB each time a part of that type is placed. This is quite a common root cause if you find that all the parts which are broken are of the same type.

author: jitter
author_rank: Frequent Contributor
timestamp: September 30, 2016, 09:06:24 pm
content: Quote from: free_electron on September 30, 2016, 06:35:47 pmFlexing caused by depanelizing is something that I have indeed seen to be the cause of cracked ceramic capacitors and resistors that were placed very close to the V-groove or the mousebites.

author: georges80
author_rank: Frequent Contributor
timestamp: September 30, 2016, 11:49:59 pm
content: MLCC cracking explained (just an example, I'm not endorsing their product...) :http://www.holystonecaps.com/PDF/TechNotes/201401161435480.CracksMLCCandsupertermsolution.pdfcheers,george.

author: jitter
author_rank: Frequent Contributor
timestamp: October 01, 2016, 07:06:38 am
content: I wasn't aware of the existence of ceramic capacitors with "flexible" terminations, so I had a look at some more well-known brands used extensively in industrial electronics.And sure enough, they market that type too, e.g. theFlexitermmade byAVX, and Kemet calls itflex crack mitigation. Andhere'squite an elaborate application note on thermal and mechanical stresses on SMDs from Vishay.

author: AndyC_772
author_rank: Super Contributor
timestamp: October 01, 2016, 09:31:30 am
content: Aren't 'flexible' terminations more about avoiding thermal stress cracks during use, than mechanical damage due to excessive PCB bending?

author: Ice-Tea
author_rank: Super Contributor
timestamp: October 01, 2016, 11:00:47 am
content: Quote from: AndyC_772 on October 01, 2016, 09:31:30 amI guess they help for both? I don't use ceramics above 1206/1210 and if I'm forced to I'll select flex therm..

author: Niklas
author_rank: Frequent Contributor
timestamp: October 01, 2016, 11:18:32 am
content: There is no bulletproof way to detect cracks on already assembled PCBAs without having a lot of false positives/negatives. I have both seen parts coming back from the customer'r production line and also parts that have been in the field for over a year. Some of them have been possible to enable/disable the short circuit just by tapping on the board's edge.The best option is of course to adjust the layout and include a 2 mm keep out zone inside the PCB outline. Also the rotation of the capacitors can have an impact. If you cannot change the layout, for instance due to customer approvals, you might be able to adjust the assembly panel to use a mix of v-cut and routing. I used to draw v-cut as a 0.6 mm wide line at 0.3 mm out from the desired outline and 2.4 mm routing at 1.2 mm out. If not specified, your PCB assembler might squeeze in you single board design edge-edge and then add the v-cut, effectively removing 0.3 mm of you designed top/bottom surface outline.

author: Pack34
author_rank: Frequent Contributor
timestamp: October 01, 2016, 12:21:14 pm
content: Quote from: Ice-Tea on September 30, 2016, 08:32:46 pmYes, a v-score. What is the safest distance? This is a really compact design.

author: jitter
author_rank: Frequent Contributor
timestamp: October 01, 2016, 01:34:50 pm
content: Quote from: Pack34 on October 01, 2016, 12:21:14 pmAnsweredhere.Relevant passage from that article:QuoteSo, absolute minimum distance from V-grove to component is 3 mm/0.119 in. You're at 25 mils or 0.635 mm...A good assembly house should have informed you if you didn't adhere to the design rules before production started. If not as a service to the customer, then at least to cover their own arses.