(Poll) Do your vias and pads ever overlap? - Page 1
Forum section: electronics / beginners

author: sparkydog
author_rank: Regular Contributor
timestamp: January 08, 2024, 05:39:33 pm
content: Since myoriginal questionhas seen no love, maybe a poll would be interesting?Comments welcomed if you think I missed an option!

author: dino7
author_rank: Newbie
timestamp: January 08, 2024, 06:15:02 pm
content: I do not like via-in-pad because of the solderwick effect. Only except for when I really want to wick away heat from some device, but then again I try to via it besides the device instead of in the footprint.

author: tszaboo
author_rank: Super Contributor
timestamp: January 08, 2024, 06:43:04 pm
content: Every single middle ground pad on QFN packages.

author: isometrik
author_rank: Regular Contributor
timestamp: January 08, 2024, 07:09:42 pm
content: There are no issues when via in pad are filled/capped, planarized, and then surface finished (ENIG) by your PCB fab house.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: January 08, 2024, 08:23:55 pm
content: None of the above:Pads and vias may partially overlap, when the via is tented and the pad's soldermask opening doesn't quite touch the barrel of the via.Recent example.As above, via-in-pad is fine when adequate solder volume is available. QFN pads tend to have a bit too much, and the wicking helps if anything -- within reason, mind. The alternative is simply dosing less, which is also done on a regular basis.Small vias wick less; <= 12 mil (0.3mm) wick slowly, especially with lead-free. During a typical reflow cycle, they might not even fill all the way through.Single-tented and blind vias are to be avoided. Flux flows into the cavity, bubbling away, creating voids. Always fully tent, or un-tent both sides, when one side is being soldered.Note that, while you can try and tent a via within a wider pad (soldermask capping), the mask adds height, worsening planarity, and it still presents an opportunity for flux to bubble away and create voids. (Flux is lipophilic, so is soldermask and bare PCB, so it sticks around.) It's also a PITA, because you basically have to draw the soldermask opening entirely by hand.Tim

author: LinuxHata
author_rank: Frequent Contributor
timestamp: January 09, 2024, 04:43:02 pm
content: Yes, I always add via, whenever possible, to the pad, because it improves mechanical strength of the pad.

author: Infraviolet
author_rank: Super Contributor
timestamp: January 09, 2024, 07:05:53 pm
content: Thinking in regard to 2 layer boards, so no blind/buried vias possible:I think some PCB manufacturers refuse to allow pad-in-via, atleast on cheaper board production options, for manufacturing reasons? Some have issues with having exposed plated copper in the same place as a via? not just because of the effect it can have at assembly stages later on.If one is hand assembling the parts on the board then one can usually overcome any solder wicking effects of the via, adding extra solder until it is full and then soldering the actual joint if it comes to that. If the manufacturer is willing to produce pad-in-via.If one has a board machine assembled then the machine wouldn't notice any wicking and that could get troublesome.

author: Faranight
author_rank: Unknown
timestamp: January 09, 2024, 09:05:45 pm
content: I regularly use via-in-pad's since I'm mostly doing hand-soldering, so I'm not worried about the solder-wick effect.Sometimes it also allows for easier placement of uC decoupling caps on the back side of the board when space is constrained.

author: Terry Bites
author_rank: Super Contributor
timestamp: January 12, 2024, 04:22:19 pm
content: I feel a via in pad is fine unless you've identified a production issue resulting from it.Keep the via size to a minimum.Via plugging keeps the solderwick fairies at bay.