Help with resources for SMT assembly

http://www.surfacemountprocess.com/solder-paste-printing-process.html this site has some pretty good general advice for the novice. I am in no way affiliated with it and I have no idea as to its eventual commerical aims (if any).

ENIG if your product can justify the cost is by far the best option for PCB finish. It is flat, clean and stores well.

Most paste manufacturers will happily send you a pot to try for free. That way you get to put the paste through your process with your products. Nobody else can really tell you what will work best for you.

There are several alloys out there for lead free process but by far the most common is SAC305. Most manufacturers will offer this in a no-clean variant. In the UK at least getting non no-clean SAC305 paste is actually often a special order requirement. I wouldn't concern yourself with looking for extreme detail on flux properties. If the paste uses the right alloy and has the cleaning poperties you require the flux will have the properties required because if it didn't it would not be fit for purpose.

Instead you should look at the properties that match how you use it. Storage, how long you can leave it on a stencil, how long you can leave it on PCB before reflow. How quickly you have to finish the pot once you have opened it. Plenty of pastes expire in 7 days or less once you break the seal.

As a small batch manufacturer we have specifially chosen a paste that once removed from cold storage (where it is happy for 6 months) can be kept at 30C for 30 days and remain perfectly useable. Henkel now make a paste called GC10 that goes even further with properties that allow you to treat paste in ways that would have half the engineers on here frothing at the mouth with apoplectic rage.

Suggestions on pastes to try depend on where you are. In the UK you can try Multicore (Henkel), BLT, Almit, DKL, Warton (By far the cheapest, but not as much information in their datasheets),Qualitek,KOKI & Cobar to name just a few. The manufacturer or representative will send you a sample in all cases. Several of them also have an engineer who will visit to give advice, point you to the correct paste, help with your profile (using their own equipment). I have no doubt the same is true in other territories.

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With regards to PCB surface finish I would recommend Electroless Nickel Immersion Gold (ENIG) because of the long shelf life, tarnish resistance, RoHS-compliance, and easy soldering compared to other surface finishes. http://www.surfacemountprocess.com/preferred-pcb-surface-finish.html

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