Then, with home brew boards I usually flux the entire board with a flux pen before applying the solder paste. (measured on the PCB) For #1, you need to make sure that your PCB is perfectly clean. I usually limit my peak temperature to about 200☌. Mechanics paste has a fairly low melting point and that flux tends to bake on if it hasn't evaporated. You also may be too hot and too long in the solder phase. You are not allowing enough time for the solder paste flux to do its job. To improve this process, I think you need to look at your reflow cycle. The problems with this are exacerbated by #1. It appears that generally you have way too much paste on the pads. In addition, to the flux issue, you may also encounter problems with tombstone caps etc. As someone else mentioned, you are not getting good flow over the whole pad. I notice a couple of issues on those boards that might help. I repeat this process several times but lay the cloth over the PCB and use a dry toothbrush to soak up the resin and IPA.
#Solder paste for reflow free
let it sit for 30-60 sec and then wipe it with a lint free cloth. I brush on IPA with a very wet toothbrush. I use IPA to clean my boards and have only rarely had resistant patches of baked flux that need scraping and then further cleaning with IPA.
#Solder paste for reflow cracked
Do you guys have any suggestions to clean this or to reduce the amount of residue after reflowing? EDIT: Last two pictures show the cracked residue after applying pressure with tweezers. some more of the flux can be evaporated, but it still leaves that nasty solid flux residue. I was also wondering if this could be due to not applying the popper amount of temperature during the proper amount of time to allow the flux to evaporate, so I reflowed the paste using a hot air instead, and well. I could try using some other solder paste brand, but maybe someone knows a proper way to clean it.
I even tried using acetone which didn't work neither. I have tried the usual isopropyl and brush to try to clean it without any luck. The thing is that I'm having trouble to clean the residue. The solder paste I'm using is Sn63/Pb37 XG-Z40 from mechanic (It's probably not the best solder paste in the world.). I have attached some pictures for you guys to see it. It is crystallized and when pressure is applied it cracks. The residue is solid and looks like solid flux rosin. After some testing the reflow process seems to be working fine, but I'm getting some ugly flux residues. I have to assembly a relative high number of boards using SMT, so I decided to give reflowing a try and purchased a cheap T962 reflow oven.
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