How to Make DIY Solder Paste for Tinning PCBs at Home

I was looking for a solution for pre-tinning my DIY PCB boards. One way to do this is by reflowing solder paste. Another very cool use is repairing brass instruments - like trumpets, trombones and tubas, because all you need to do is heat up a joint with the paste in place and it magically bonds at the right temperature.

If you go on EBay and search for solder paste - the pro stuff costs an arm and a leg for small amounts, so I was wondering if it is possible to make entry grade solder paste at home. After looking at several forums, I found a forum conversation in which someone used shavings from a filed block of solder mixed with solder flux to create paste, and reported it as success. I decided to recreate this, and in the process discovered that it is much easier than i thought. And the bonus is that working with the pre-tinned pcbs is so much easier, soldering times are now significantly reduced. 

WARNING: THIS PROJECT INVOLVES FILING SOLDER - WHICH IS AT LEAST 40% LEAD. WORK IN WELL VENTILATED AREA AND WEAR A FACE MASK AND GLOVES. ALSO MAKE SURE LEAD DOES NOT END UP IN FOOD. 
Also read this: http://en.wikipedia.org/wiki/Lead_poisoning

What you will need:
1)  Solid Solder - 50-50 or 60-40. You can use solder that has flux in it - as long at it is NOT acid based flux which will corrode your components
2) A medium to fine file - the finer the file - more work but better quality paste
3) Solder Flux - also called "solder paste" but do not confuse this with real solder paste. Make sure it is not acid based for intended for soldering! Radio shack sell some of this stuff. 
4) a toaster oven, fire, or oven. 

This instructable has 12 steps.

1) Cut the solder into strips or pieces
2) Put in aluminium foil. fold the foil a few layers thick so no solder will leak out and mess up your nice toaster oven or stove or leak on something important like your foot
3) Form a makeshift "pan" or "boat"  

You want to melt the solder into a nice big blob.

I used the toaster at the highest heat (broil setting) for 40 minutes.
You can also place the boat on a metal plate over a counter top stove. WARNING:  Do not put the makeshift boat/pan directly on a fire, as it will eat through the aluminium, and you will have a nice solder puddle to clean up. You do not want to get in trouble with the local home management team. 

Once the solder melts, take it out/off and let it cool down into a bar of solder, the shape does not really matter. 

Discard the aluminium wrapping. 

Note: MAKE SURE YOU GET RID OF ALL THE ALUMINIUM FOIL AS IT WILL END UP IN THE PASTE IN THE NEXT STEP!

Simple really: File or grind the solder bar into a fine powder. Note if you go fast you will get bigger pieces and eventually the bar will heat up, so you want to rotate the bar from time to time to prevent it from melting due to the friction heat!

MAKE SURE TO WEAR PROTECTIVE GEAR. OK SO I AM NOT WEARING GLOVES. DOES'NT MEAN YOU DON'T NEED TO!

In this picture i added too much powder to the flux and it became chunky - I will need to add more flux to this paste. Nevertheless, this is sufficient for the next demonstration.

After a few test boards - I decided to try this out on a real project. For the purpose I revamped a classic NEVE based preamp that will be transplanted inside a classic RCA Varacoustic ribbon microphone, and will give it the gift of a super sound, phantom power and real world versatility. 

I was in a rush to show off, so unforunately I didnt strip off all the photoresist (the blue residue on some of the pads and leads. This is where the solder will not catch on properly. Lesson learned. Next time let the board sit in the resist remover (baking soda)  instead of trying to quickly scrape it off. 

I put what i though was a little bit of paste on board. When you see the results you will see I should have put less. and spread it around. It really doesnt matter where the gray solder is. once the flux melts and the solder melts - the solder will magically be pulled onto the traces and coat the copper. 

TIP: One nice thing I discovered is that if you want to get really good results in etching, exposing, and tinning - you can clean off the board with Comet kitchen cleaner and it works better/safer/quicker/far-better than acetone. 

For this demo I used a hot air rework gun so you can see it in operation. You can also use the "toaster reflow" method if you can get your toaster to 500F or try the broiler - also might work!

Here i stopped half way - just so you can see how the paste magically flows onto the traces

After the board is completely flowed - it will be covered with flux which you will need to clean using (yes again) Comet - or dish soap and water. You can use scotch brite to get rid of the flux. 

As you can see for a first try its not bad - no solder bridges!  Assembling a PCB becomes a piece of cake. You can also use it for SMD components (yes I tried - there are a few SMD Caps on the board that were easy to assemble).

The final results are a cost effective way to produce enough solder paste to last you a lifetime, at home with minimal effort!

Hope you enjoyed this instructable!