I'm going to have 10 PCBs, each 6x1" and no more densely packed with chips than an Arduino, and I'm trying to determine if I need to go with a $42, 250g jar, or if a 35g syringe would be sufficient. There's no reference for scale in the photos on Digikey but the syringes seem tiny, like those ones you get with heat sink paste in them. Yet Sparkfun sells a 50g lead-free jar for half the price of even the syringes on Digikey, so perhaps those photos are misleading. (I really wanted to go with leaded as I hear lead-free can be difficult to work with.)
How are you going to apply paste?
scswift:
I'm trying to determine if I need to go with a $42, 250g jar, or if a 35g syringe would be sufficient.
That depends on how neatly/sparingly you apply it...
I am going to use a stencil.
Syringe - you don't want to be landed with 250g that goes off. The solvents
evaporate with time so once a jar is opened the life is limited. I'm on my
2nd syringe in 4 years of making boards...
Start with 0.15mm or so stencil thickness if uncertain, thinner will be
better for fine parts.
Hm, I found this page with photos of some solder paste syringes that gives a good indicator of size:
So these are quite a bit larger than the tiny ones they give you for heat sink paste. And it doesn't seem so terrible doing it by hand without the stencil. My parts are all 0.65mm pitch, and that guy is doing 0.5mm.
I should probably get one of those suction devices for placing the chips.
35g should be plenty.
JohnLincoln:
You need to keep it refrigerated, in order to maintain it's shelf life. but i wouldn't recommend keeping it with your milk, cheese, butter etc. unless it is very well wrapped.
Should issue the general warning here, which is that volatile solvents and fridges
do not mix. However the amount of evaporation from solder paste in a lidded jar
is probably OK.
The result of careless mixing of solvents and fridges is a large explosion when the
door-switch for the internal light causes a spark in solvent-enhanced air. Some
solvents are very volatile even at low temperatures.
I went thru these sites last time I ordered to find a place with best price.
I ended up with solder from CML Supply. EP256.
Small needle great for putting a very small amount on a pad that was missed
or for touchup.
Solder paste in syringe keeps a long time. I just keep mine on the workbench. Solder in the needle end keeps one sealed, the syringe itself keeps the other end sealed. Have not seen any poor results from seperation.
35g syringe will do the 10 cards easy. I use a wider needle for applying paste across the narrow width of the board in a few places so a huge blob of solder does not need to be dragged the full length of the board - I think that helps with keeping solder use down also. Does lead to missing a pad every once once in a while on a part that seperated from others on the board. Can touch up if you don't see it until you're placing parts, or just wipe the board off and squeegie it again.
Make sure to wipe the board down with alchohol before applying paste.
Larger tip for putting out a bigger amount across the board to squeegie:
http://www.all-spec.com/products/weller/soldering_and_rework|soldering.rework_accessories|sol-20/KDS22TN25.html
Might even have cut it down some so the opening is a little bigger.
I use a 1" stiff plastic putty knife from the hardware store.
3mil mylar stencil from polulu.com for fine pitch ICs.
4mil if pins are wider to get a little more paste on the board.