Friday, July 6, 2018

How to tin your PCB cheap, fast and safely | Homemade PCB

The last step in making your own printed circuit board (PCB) is tinning. Tinning is important because copper oxidize very fast, leading to an ugly look, hard to solder pads and over longer time, breaks in traces.
When you order PCB's from a manufacturer, the pads look all nice a shiny. I was looking for a way to achieve this at home and I found one. It doesn't involve strong chemicals, it's cheap and it only takes a few minutes.

DIY PCB tinning at home using plumber's paste
PCB after tinning
PCB before tinning
Images were taken using a scanner, so they may look a bit funky. They look better in person. I had many of this boards from experimenting with an UV exposure box for making PCB's and so, some tracks have breaks in them or shorts. The shorts are not caused by the tinning process.

PCB tinning at home using plumber's paste

The paste that I use is Cu-Rofix 3 and contains 60% Sn97Cu3 and water soluble flux.
  • First, the board must be scrubbed until it locks shiny, using something not very abrasive like a scouring pad for cleaning dishes and then cleaned with rubbing alcohol.
  • Next step is to apply a thin layer of paste on the exposed copper. The paste can be spread using a plastic card or a piece of PCB, until no copper can be seen.
  • Now the paste must be heated. I used an electric hot plate and using some pliers I hold the board over the hot plate at about 1 - 2 mm distance with the paste facing upwards. A hot air gun would be better I think. After a few moments the flux will make the paste liquid. Apply heat until the paste is dry.
  • Remove the heat and wait a few seconds for the board to cool and remove the paste using a paper towel.
  • To remove the flux, wash the PCB in water at room temperature or colder while scrubbing it using a toothbrush. After that, using the brush and paper towel clean it using rubbing alcohol. I will explain later why this is important.
  • At the end, the copper will be tinned with a thin layer of a few microns thick.

Things to consider

Since the containing flux is not meant for electronics, it attracts water from the air causing the clearance between tracks to become conductive with a resistance starting from a few tens of mega ohms. In most cases this is not a problem but is analog circuits or when high voltage is used it might be and that's why the PCB must be well cleaned using rubbing alcohol. Water is not enough.

To test if the flux is gone, use the highest range of an ohmmeter and probe between closest traces and between traces and ground plane. Notice the resistance. Now blow on the board from a distance of about 10cm like you are trying to fog a mirror and notice the meter. If the PCB is cleaned properly, the meter should indicate OL (Open Loop). If there is still flux residue, the ohmmeter could indicate a resistance starting from 2 - 3 mega ohms and slowly increasing over 20 mega ohms as the water evaporates.

In case you find a short circuit, most probably the board was not etched or exposed enough. Once I had two traces shorted by a very very thin piece of copper that I barely was able to see using a magnifier glass.

In the above pictures the clearance between traces is 0.25 mm and the ohmmeter indicates OL, no I don't think it will be a problem for analog circuitry. My multimeter only reads until 20 mega ohms.
Using a solder mask I think could improve things.

Heating time

After the paste locks dry, you should be able to wipe it off using a paper towel. If the paste is very solid and requires hard objects to remove then you over heated. If the whole board is a big blob of solder then you really over heated. Keep the propane torch away.

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