Solder bridges under 0805 packages - Page 1
Forum section: electronics / projects

author: george graves
author_rank: Super Contributor
timestamp: March 20, 2014, 08:15:22 pm
content: So....I'm messing around with some SMD solder - reflowing in a toaster. I usually don't have any issues. Yes, my paste is a bit old, but I'm getting some really odd things happening.I'm very carefully placing the components on the board, so I know I'm not physically disturbing the paste. I'm using a stencil to apply the paste - and I inspect it before placing the componets, and the paste looks great. Nice neat little pads of solder paste - no bridges, or anything.But it's almost as if the solder is wickking under the components. (in this case 0805). I'm using a home etch board, so I know the lack of solder mask is working against me....but it should still work, right? I mean, the solder is doing this crazy thing all on it's own. Like it's trying to grow roots or something!!!!What could cause this? Not enough soak time? Old paste? Any ideas?Here a pic of the underside of some 0805 caps. I used hot air to remove them, flipped them over and snapped a pic.

author: george graves
author_rank: Super Contributor
timestamp: March 20, 2014, 08:36:22 pm
content: Here are some better pictures.

author: David_AVD
author_rank: Super Contributor
timestamp: March 20, 2014, 09:03:44 pm
content: Too much paste? Got some pix of the still soldered down to the board?

author: apelly
author_rank: Unknown
timestamp: March 20, 2014, 09:20:51 pm
content: Looks like a flux issue - old paste

author: Dave Turner
author_rank: Frequent Contributor
timestamp: March 20, 2014, 09:44:34 pm
content: Possibly a temperature or cleanliness issue!

author: nanofrog
author_rank: Super Contributor
timestamp: March 20, 2014, 09:57:55 pm
content: I'm thinking the old flux component of the paste is the biggest culprit, though it could be combined with other issues mentioned.

author: ablacon64
author_rank: Frequent Contributor
timestamp: March 20, 2014, 11:09:19 pm
content: I had some old (almost dry) solder paste, put some IPA, mixed and it worked just fine. I don't know if tin (or other solder alloys) has an expiration date. I think it's only the flux that gets old (I'm ignoring oxidation here).Okay, let me explain. I know solder comes with expiration dates printed but a couple of years ago I bought 4 solder balls pots for BGA rework (leaded 63/37 alloy 0.5mm and 0.6mm) on ebay, they are just tin, no flux at all, they are expired (more than one year) and still working pretty fine. As long as there is no oxidation, it's fine.Maybe the rise temperature curve is too fast? Although you have pointed "I usually don't have any issues"...

author: Psi
author_rank: Super Contributor
timestamp: March 21, 2014, 05:14:47 am
content: +1 for flux issue.With a home made PCB + old paste you have a worst case scenario. The raw copper is covered in oxide/oil and the paste has little to no flux left. When the paste reflows some of it is balling up instead of bonding to the metal surfaces.Another thing to check is that you actually etched all the unwanted copper off the PCB.Have a look under magnification, if there's tiny specs of copper left between the 0805 pads the paste maybe sticking to it and forming the bridge around that point

author: george graves
author_rank: Super Contributor
timestamp: March 21, 2014, 06:49:24 am
content: Silly me. I think I solved it. I did another run of boards - and all was fine. At least so far.Not an etching issue - I've gotten really good with that after switching to 40% peroxide. Not a cleanliness issue, I sand with wet/dry 500 grit paper, and clean with IPA or acetone just before applying the paste. Clean as can be.I feel silly for this, but I *forgot* to mix the paste. It looked like it was homogeneous mixture when I was applying it by a small spatula. But I had stored it outside the fridge over two nights, and that caused the flux to separate from the lead way more then if it had been stored cold. So I was working with a huge flux to lead ratio. I think there was so much flux that it was "bubbling" under the chips in a little volcanic explosion - causing those "roots" to grow.That and a "turn on the toaster oven and watch till it flows" method made it worse. Too much heat too fast - no "flux action" period to let the flux to just sit there and clean.....and partially burn off. Not to mention no solder mask. So the excess flux would "sizzle" and push lead under the chip.That's my theory anyways. I'll post up if the problem isn't truly solved.(a side note - I'm impressed by my little sony video camera to get me those images!!!! Yes - that was a HD camera, with the stock lenses! Right click, and view image, then zoom in)Thanks all. I was really scratching my head at that one.....