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Too much paste is an obvious possibility, but in my experience the biggest factor is inadequate flux. You can pre-flux the pads with a brush before applying paste/parts, which will help immensely. Proper pre-fluxing is essential for any BGA work.
FWIW, Solder paste is essentially powdered solder suspended in flux, and the balance is usually right, but as it ages the solution becomes less homogenous and you might get inconsistent results, which is why they have expiration dates on them (and why they recommend refrigeration).
Stencils will obviously help by dispensing the exact right amount of solder, but they're not always economically feasible for small projects. Good fluxing is a cheaper first step. But as the previous commenter mentioned, if you have a lot of fine pitch parts, a stencil is worth it if you don't want a 0% yield on first pass.
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I hadn't heard of that - I will have to try pre-fluxing today. Flux certainly helps clean up bridges afterwards.
Don't use too much, or you'll have a mess and it'll cause problems with boiling; a thin film is all you need. When I reball FPGAs (obviously a different application without any built-in flux), I scrape it on with an old credit card.
there are lots of reasons this could be happening. the most obvious is too much paste. placing paste with a needle can be pretty difficult to get right, specially if you are doing it by hand and not using a dispenser unit. if you have a lot of IC components with fine pitch, it's far better to spend some money on a stencil, which will provide much better results...
Notice the bridges on the 8-pin IC in the lower right.
I use a skillet for reflowing PCBs. It usually works great. Today, I got these bridges on a fine-pitched part. Is this due to too much paste? Old paste? Improper pad design?
I'm thinking too much solder - I put it on with a 22ga hypodermic needle, which was really too big for these pads. Usually, when I overload with paste, I just get solder balls on the PCB surface.
The bridges clear right up with a dab of flux and a swipe of the soldering iron, but I don't want to have to do that every time.
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