Cleaning Solder Paste Misprint

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.

Jim
This comes from M.H. and it has to do with post print inspection which 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.

Never clean misprinted paste by "scraping it off" or wiping, as you will end up with embedded solder fines between edge of mask and edge of pads, as well as solder paste squeezed into vias, etc., especially if using Type 4 paste, which is quickly becoming the more common type used in the industry.

Please refer to IPC-7526. A special batch cleaner (often called a dogwash) is used for the purpose of cleaning misprinted paste from PWBs, and can also be used to clean stencils with the right spray/filter setup. It will gently wash away the paste on either side, and then after a pass through your normal cleaning process and a short bake you can then re-print with a clean, dry CCA. They are now made with dual solvent capability so you can clean with either a semi- aqueous solvent wash for RMA/no-clean paste, or an aqueous wash only for water-soluble paste. Check out Stoelting/UnitDesign, they can help you with this.

The Assembly brothers rarely get it wrong, but in this case they need to review cleaning 101. When attempting to reclaim a mis printed assembly, one should never attempt to scrap of the excess solder paste. That is the worst thing you can do because it pushes the solder paste into tiny spaces like pad channels and vias that are impossible to clean.

The same goes for wiping, rubbing, squeeging, itching and scratching the solder paste on an misprinted board. Never do it! The right thing is to do to reclaim it is to follow these steps.
1. Do not wipe it!
2. Put it in an automatic (wash , rinse and dry) stencil cleaner. An air spray system is preferred over ultrasonics because of cleaning and rinsing advantages.
3. Visually inspect the board to confirm it meets standards.
4. Baking is recommended to prevent assembly defects due to excess moisture.

Finally, in this case,don't clean like my brothers!

Speedline Electrovert have developed a feature that provides the capability to deflux the bottom side (reflow side) and clean the mis-print side at the same time. The feature is designed to prevent solder spheres from being re-applied onto the board assembly. The mis-print cleaning feature is available for both batch and in-line cleaning applications.

We follow IPC-7526 Stencil and Misprinted Board Cleaning Handbook.

We manufactured a fixture to suspend our board with the paste side only touching the chemistry used to clean the solder paste and turn on a ultrasonic system for 4 minutes. We then wash the PCB in our water wash system and prior to releasing the PCB back to production we vacuum bake the board at 60 degree C for 15 minutes to remove any moisture from the board.