Phil
Welcome to Board Talk. This is Jim Hall and Phil Zarrow the Assembly Brothers, also by day known ITM consulting, here to solve your process and SMT related questions. What's today's question?

Jim
This is an interesting question coming to us from M.H. and it has to do with post print inspection. This is close to our hearts because it is something we always stress with our customers and have had tremendous success helping our customers implement 3D post print inspection for the improvement of overall quality.

This question goes beyond the inspection process and asks, "What do you recommend we do when we find defects in the solder paste print on the second side. We currently do not have an adequate cleaning process and scrap the boards when we find second side pasting defects."

So what is he saying. They have already reflowed components on the first side, what we call the bottom side. Now he is printing the second side and has a mistake on the print on the second side. He is saying they throw those boards away with all those valuable components?

Phil
Even if they were inexpensive boards it's a horrible waste of value added on the second side with components, time and energy invested. It's not necessary. It is a question that people wonder how to clean first side and second side.

We see a lot of improper cleaning of first side as well. The first thing you want to do is scrape off all the excess solder paste with a Kim Wipe using some isopropyl or approved solvent.

Jim
Don't assume any solvent will do. You've got to use a solvent that is compatible with chemistry of the specific solder paste you are using.

Phil
This is where it gets a little tricky. A lot of people stop right there. If you were to examine the board under high magnification you would find that there is solder paste now diluted trapped between the solder mask and the pads, in vias. So how do you get it out of those little cracks.

A while back we did a lot of experimenting with this situation and we found the best way to clean solder paste misprints is to run boards though a water cleaner. If you don't have a water cleaner you may wish to use an ultrasonic cleaner.

Jim
Referring to the original question where M.H. states that they don't have an adequate cleaning process, perhaps a simple economic analysis of the value being thrown away might justify a cleaning process.

Phil
This has been Phil Zarrow and Jim hall the Assembly Brothers and remember ... 

Jim
Don't solder like my brother.

Phil 
And don't solder like my brother.