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Reflowing without Solder Paste (March 18, 2008)
I've discovered a neat trick! Using (at least one manufacturer's) Lead-Free PCB finish, it's possible to reflow solder without the solder paste!
The PCBs were ordered from Advanced Circuits with their SMOBC finish plating and I can reflow components onto them in a regular toaster oven set to ~475/500°F. I've attempted the same task with some Gold Phoenix PCB with plain old Pb finish and had no success.
So, share and enjoy, try it and let me know how it goes. Worst case scenario, you heat some chips and boards.
Update (hours later)
The PCBs survived several hours on the vibration/earthquake table, but I am able to remove the chips with sufficient sideways pressure (I can't remove the pasted chips). While I wouldn't expect failure under normal circumstances, I couldn't recommend this as a production technique, even for small time work. It could however, be used as the Post-It note of reflowing. The adhesion is good, but not too good you can't scrape it away and try again.
With the oven cranked up to 550-600°F (using a variac to surpass the oven's 450°F top end), I was able to reflow a Gold Phoenix PCB.
Reflow (1)
Solder (2)
Posted by spiffed at March 18, 2008 12:18 AM
Comments
brush paste flux across all the pads on the board.
Take a broad tipped chisle point iron, and push a 1/8" to 1/4" solder ball across all the pads. The solder ball will leave a small mound of solder on each pad when done right ... do not allow a huge mound, or work at getting too little left.
Clean the board, and again brush all the pads with paste flux, as well as the leads/balls on the parts you are going to attach. Place the parts on the pre-tinned pads, place board in your oven, in your skillet, or on your hot plate at a temp about 20 degrees F hotter than your solder melt point, and below the critical temp of any part. If using a skillet/hotplate, use two ... one set just below the melting point for about a minute, and one just above melting point for a minute (things will heat fast and reflow) and remove back to a surface that is fairly warm to allow the solder to solidify and parts to cool uniformly without huge thermal stresses.
This works great for home DIY FPGA's and CPU's with BGA attach.
Use water soluable paste flux for easy clean up with mild dish soap, and air dry.
Posted by: totally_lost at May 30, 2008 6:43 AM