Applying paste to a fine-pitch BGA-esque connector pad without a stencil? - Page 1
Forum section: electronics / general

author: PfhorSlayer
author_rank: Contributor
timestamp: May 19, 2023, 07:02:38 am
content: Hey all, I've got a project that involves a pair of Samtec's ADM6 (https://www.samtec.com/products/adm6, the 60-row versions) connectors on the board. Unfortunately, after reflowing the rest of the board, one of the connectors was found to have reflowed incorrectly and bridged a bunch of the pins on the inner rows (which are inaccessible). I do happen to have another salvaged connector, but since the rest of the board is populated and reflowed, I am unable to use the stencil to add paste to the connector's footprint.Note that the connector itself has solder balls (basically a BGA!), but they melt at a much higher temperature than the 183C solder paste I am using. I do not want to melt these solder balls if at all possible (ideally the low-temp paste will melt onto them and make the connection), as that could potentially leave me with another broken connector if any bridges happen to form internally. These connectors are quite pricy ($25 a pop!) and I'd rather not destroy any more of them.What are my options here as far as reflowing the new connector onto the board once I've removed the old one and cleaned up the pads? Is there a way I can get an even amount of paste on each of these 0.365mm pads without the stencil? Other options?

author: jpanhalt
author_rank: Super Contributor
timestamp: May 19, 2023, 09:24:16 am
content: How crowded is that area? Individual stencils for BGA are available, e.g., here:http://www.proto-advantage.com/store/index.php?cPath=4000&osCsid=b6irv1s7kenvontq321iqrpco2If no stencil will fit, then you are left it seems with applying dots. A flat tipped needle is what we used to apply reproducibly microdots of liquid*, but not solder paste. Of course viscosity and rheological properties of solder paste are different than the liquid we were using. You will likely need to thin the past with flux.Doing it by hand will be a nightmare, unless you are really steady. A mechanical aid will help.*Today, I use that method to apply cyanoacrylate adhesive to PCB's.

author: Matt-Brown
author_rank: Regular Contributor
timestamp: May 22, 2023, 01:35:41 pm
content: You could remove the solder from the balls on the conenctor, make a small fixture to hold the connector ball side up and reuse your stencil to apply paste to the connector and not the board.We have done that with 0.2MM pitch BGAS

author: SMTech
author_rank: Frequent Contributor
timestamp: May 22, 2023, 02:33:59 pm
content: clean up the area. dab/dip the connector in some sticky flux and let the balls do their thing, the trick will be getting the connector in the right place.

author: PfhorSlayer
author_rank: Contributor
timestamp: May 24, 2023, 06:03:38 pm
content: I spent some time attempting to manually apply solder paste to each pad, which seemed like it would work, but I ended up over-doing it and bridging a bunch of the pins once the connector was reflowed. Bummer. :|The connector itself has alignment pins, which make it super easy to put in the right spot on the board, but the stencil obviously does not have holes for those pins which means I can't really use the stencil to apply paste to the connector itself in any meaningful way.I've got some new connectors coming, so I think I'm just going to try the last suggestion of skipping paste altogether and just letting the solder balls do their thing. My only concern is that since the solder balls are lead-free, I'm going to need to heat the board up pretty significantly to get it to reflow (versus the 183C paste I'm using, which tends to reflow nicely when I've let the board sit for a moment at 200C on the hot plate). Perhaps my best bet is to crank it up to ~260C and then add some 350C hot air to hopefully get things moving a bit quicker?