Tarnishing is a chemical change on the surface of a metal due to compounds in the air. Tarnish can form on a printed circuit board, on the contacts of a switch, on the pins of a part, or on connectors. Tarnish makes soldering more difficult and reduces electrical conductance.
Copper is moderately difficult to solder and it oxidizes fairly quickly, so copper is usually plated with tin, nickel, lead, silver, gold, or some combination of those elements. Even so, oxides, chlorides, sulfides, and other compounds eventually form on plated surfaces that are exposed to air.
Hobbyists that make their own PCBs usually don’t plate their boards (although tinning kits are available), so homemade bare copper PCBs are even more vulnerable to tarnish than commercially plated boards.
For optimal soldering, printed circuit boards and other parts should be soldered shortly after being received from the manufacturer or being etched. Boards and parts should be stored in sealed containers to keep them clean, undamaged, and to prevent tarnishing.
While I try to keep parts in a plastic container, my PCBs are often subject to a bit of abuse. For example, I often make several different circuit boards on the same PCB panel. That means the individual boards need to be cut apart -- usually with the accompanying dust, machine oil, scratches, and fingerprints of machining. The same issue occurs if the printed circuit board needs to be machined into a circle or cut to an unusual shape to fit into a device.
Dirt and oils need to be removed from a circuit board before soldering. Soap and water, isopropyl alcohol, and/or commercial electrical degreasers are required to perform this task. This article focuses only on removing tarnish from the metal surfaces of a board that has already been cleaned.
Using an eraser to remove dirt or oil will just as likely smear some of it around and grind some of it in. And, those contaminants can be transferred by the eraser to another board. However, in a pinch, you can get away with using an eraser for cleaning. Be sure to rub off the tainted portion of the eraser onto another surface (like a piece of paper) as part of the process.
Sometimes it will take a couple of weekends to completely populate and finish soldering a large or multi-part board. And, most circuits include holes and pins for debugging and expansion that will tarnish while the circuit board is in service. It is inevitable that you'll encounter some tarnished boards and contacts.
This article takes a closer look (literally) at the most common way of detarnishing a circuit board contact -- using a pencil eraser.
I’ve used a rubber pencil eraser for removing oxidation from metal contacts for many years. Despite the effectiveness, I’ve always felt a little guilty about doing so. It seems like a hack.
Erasing tarnish from a circuit board with a classic red pencil cap eraser.
With a little gentle rubbing, you can quickly see the metal brighten up. And, you can see the eraser darken. So, something is definitely happening. But what? Is it a chemical change? A glossy coating? An illusion?
On the left, a tarnished PCB pad and hole magnified 60 times. On the right, a bright pad has been cleaned with an eraser.
Using a QX3+ digital microscope at 60x magnification, the difference between two equally tarnished pads becomes apparent after one pad is rubbed with an eraser.
The horizontal scratches occurred during manufacturing or storage. The eraser was rubbed vertically so that any scratches due to the eraser could be differentiated.
As you can clearly see, a pencil eraser scratches the metal. Therefore, it appears that the pencil trick works through abrasion. That is, the tarnished metal is physically removed. This should come as no surprise to anyone that has rubbed through a piece of paper after multiple erasures.
The eraser scratches don’t appear particularly deep. On circuit boards (as opposed to connectors), the purpose of the plating is to improve soldering by preventing oxidation and other forms of tarnishing, and to improve solder adhesion by presenting a compatible metal. After soldering is complete, most PCB plating serves no further purpose. So, in most cases, there would be no apparent harm if some of the eraser scratches breach the plating to the depth of bare copper, as long as soldering commences shortly thereafter.
But, scratched plating is a concern for connectors because they can be attached and detached multiple times. Connectors aren’t going to be covered with solder. Therefore, it would be beneficial to determine the type of eraser that produces the least amount of scratching.
Abrasion produces particles, gunk, and grit. Anyone who has ever used an eraser knows that residue is created in the erasing process. Residual material on the board surface may interfere with soldering or electrical connectivity.
Microscopic particles of eraser residue appear on circuit board traces and pads.
Surface contaminates can result in poor adhesion and weak solder joints. Will these eraser bits burn off during soldering? Or will they form a barrier? Do they release fumes or will they corrode the metal?
What is a simple way to remove the eraser bits? Is the old “brush-your-hand-across-it-while-blowing” method adequate?
Although rinsing and gentle scrubbing is probably required after using an eraser, it would be nice to find an eraser that leaves the least amount of residue to begin with.
On the next couple of pages, we'll take a look at a variety of commonly available erasers to determine which scratches the least and leaves the least amount of fine residue. We'll also investigate what level of clean up is necessary. Finally, we'll see if some other methods (besides erasers) may be effective at removing tarnish from circuit board pads.