PCB Vias Question - Page 1
Forum section: electronics / projects

author: dexters_lab
author_rank: Unknown
timestamp: December 09, 2014, 07:50:39 am
content: i have just taken apart a consumer car stereo and noticed something with the vias on two of the PCBs inside.On a couple of boards all of the vias have a separate blue mask (like solder mask) over themOn another board they have applied small epoxy blobs over one side of 8 specific viasI have attached pictures of bothwould be interested in knowing what the purpose of these two processes is for?

author: Precipice
author_rank: Frequent Contributor
timestamp: December 09, 2014, 08:14:00 am
content: The red ones look like test dots for glue, to prime the pump before it's used for actual parts - sometimes the glue sucks back into the needle if it's not used for a while. Or they could also be a board version / ID, but I'd expect them to be in a silkscreen feature if that was the case. I don't think they're particularly over vias.The blue ones, less sure. Are there any other uncoated vias on the board? There's a faint possibility that gloop is conductive, and being used to tie the layers together on an otherwise unplated board. Or it could just be very keen via tenting, automotive design rules have variously called for no and full tenting over the years.

author: dexters_lab
author_rank: Unknown
timestamp: December 09, 2014, 08:35:03 am
content: it's an aftermarket stereo from circa 10 years agowith the blue mask, it's on every single via, but the two boards that have this are part of the front panelthe epoxy/glue blobs, each one is over a via and looks quite specifically done, it is a double sided board though and this would be the glued side. Maybe they just used them as test spots?

author: Opticalworm
author_rank: Contributor
timestamp: December 09, 2014, 09:52:31 am
content: Do you have pictures of the other side of the boards?

author: envisionelec
author_rank: Frequent Contributor
timestamp: December 09, 2014, 01:53:04 pm
content: As a former Sony warranty repair tech, I actually feel qualified to answer this!The blue dots are via masking/plugging because of the sometimes corrosive nature of the automotive environment. In certain regions, there is enough pollutants in the ambient air that condense on the warm PCB in a car radio. Since vias are often unmasked, the moisture tends to collect in these openings and begins to corrode the copper growth that makes up the via connection.The red dots appear to be a "misfire" from the SMD adhesive applicator. The adhesive is used to hold SMD under the PCB during wave soldering. The selective applicator might have had some air in the line that was not properly purged. Are you certain the red blobs are on vias?

author: T3sl4co1l
author_rank: Super Contributor
timestamp: December 09, 2014, 03:33:08 pm
content: I was under the impression that vias of that style are mechanically swaged, rather than plated -- they're used in cheaper stock (e.g. glass phenolic) that can't be plated.Tim

author: c4757p
author_rank: Super Contributor
timestamp: December 09, 2014, 03:39:32 pm
content: Quote from: dexters_lab on December 09, 2014, 08:35:03 amThose are almost certainly not all over vias...

author: free_electron
author_rank: Super Contributor
timestamp: December 09, 2014, 04:41:03 pm
content: the dots are lot code / test markers. squirted on to identify production runthe blue via's are polymer based . there is no copper in the hole. they squirt this blue gunk in there which contains a conductive material.

author: marshallh
author_rank: Unknown
timestamp: December 09, 2014, 07:10:59 pm
content: Yup, it's conductive epoxy. It's the alternative to plating.I bet if you look elsewhere on the board, you'll find it was made by Mitsumi. Sony contracted out many things to them in the 90s.

author: dexters_lab
author_rank: Unknown
timestamp: December 09, 2014, 08:31:57 pm
content: right, just looked at these pcbs againthe red epoxy blobs, are not over vias as i thought, some are but i think thats just an accident so looks like these are a mistake or test or something else.the blue coating on vias, is not conductive. given it's only present on the pcbs that are located on the front panel it seems likely it's there for environmental protection as envisionelec suggests.

author: wraper
author_rank: Unknown
timestamp: December 09, 2014, 08:41:00 pm
content: I've seen the same blue dots in very cheap Samsung computer DVD drives. Those PCBs seemed to be phenolic rather than FR4.

author: free_electron
author_rank: Super Contributor
timestamp: December 09, 2014, 09:47:13 pm
content: Quote from: wraper on December 09, 2014, 08:41:00 pmcorrect. plating on phenolic doesnt work. so you squirt conductive epoxy.