A Convenient technique for Coating PC Boards with Solder

When you look at commercial printed circuit boards, the copper traces are usually coated with shiny solder or a green "solder-mask". Bare copper is the mark of a really cheap board or a homemade one. So how can you make your boards look more commercial? Solder-mask is beyond amateur capabilities. It is commonly applied by screen printing or by dry film photoresist techniques. So what else is there?

There are a group of similar "electroless plating" or "dip plating" products on the market. You dip the board in them for a few minutes and the copper traces are coated with a bright (but not shiny) coating of tin. The boards look great. Unless you leave the boards sitting around for a few weeks and try to solder them. That's when you find out that tin coating does not mean necessarily solderable coating. A study some years ago by the Tin Institute (an industry research organization) concluded that the only coatings that retained their solderability over long periods of storage were electroplated and reflowed tin/lead solder or tin/lead solder applied by hot dipping or roller coating. The "electroless tin" coatings were too thin and too porous to resist oxidation from the air for long. What I wish to describe below is a simple technique for applying tin/lead solder to the traces of homemade pc boards with nothing more than a soldering iron and a bottle of liquid flux.

First clean the board thoroughly as you would before soldering the components in place. Then coat the board completely with a generous application of liquid rosin flux (preferably RA or RMA grade such as Kester 1544). The key is LIQUID flux. The flux should remain wet during the procedure or the coating will be much more difficult.

Using a soldering iron preferably with a wide, wedge type tip, apply some 60/40 solder to the tip and touch the iron to one of the copper traces on one end of the board. Then drag the tip of the iron along the trace. Drag the iron slow enough so that the solder does not freeze but fast enough so that the trace will not be overheated. About a half to one inch per second is about right. The solder will be applied to the copper trace in a thin shiny coating. Apply more solder to the iron as required. When you come to a hole in the trace, pass the iron around the hole rather than over it, to avoid blocking the hole. Continue in this manner over all the traces and other bare copper on the board. After a few minutes practice you will get quite proficient at coating the traces in this manner. Coating a modest sized board with solder in this way will only take a few minutes and the end result will be very professional looking.

If you are going to be assembling the board immediately, this coating process can be combined with the component soldering. Load the board with its components, bending the leads over to hold them in place. When all the parts are in place, coat the bottom of the board with liquid flux and proceed to solder the component leads. But instead of lifting the iron from one pad to the next, drag it along to the next pad as in the procedure above. You will have a similar appearance to the process shown above.

After finishing solder coating all the exposed copper traces, clean the board with an appropriate cleaning agent to remove the rosin flux. Some people use alcohol but commonly available alcohol products have serious drawbacks. I have found the best cleaning material to be a paint brush cleaning product called "Poly-Clens". Cleaning with Poly-Clens is described in another document on this site.