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The Impact of Copper Dissolution on PTH Assembly

23rd September 2014

Plated through-hole - also called thru-hole - assembly, or PTH assembly, provides a method for a connection between printed circuit board (PTH) rack layers.

The principle is to drill or punch holes through the board and then plate, or metallise, them. This is a more expensive method for binding circuit board components than in straightforward surface-mounted technology (SMT), but the binds are mechanically far stronger.

The main advantage of PTH assembly over SMT technology is that, in the latter case, the soldered joints will tend to contract over long periods of time. This is an especially difficult problem in large capacitors and electrical installations such as those used in power generation that may be subject to large mechanical stresses.

PTH assembly faces a number of critical reliability factors. The circuit board material and thickness will determine the power and speed of the drilling machine used to create the holes. If the drilling speed is too great, it could cause rough edges around the holes or a breakdown of the drilling bit.

However, like SMT technology, PTH assembly still has to use solder. Since lead-free tin-copper alloys were introduced into electronic circuitry, copper dissolution has become a major issue in the bonds as well as in the hole plating.

Copper dissolution is a normal phenomenon that occurs on the contact of copper plating with molten tin-rich solder. The copper plating effectively dissolves into the solder. This dissolution is a necessary process for the creation of solder joints.

The dissolution process becomes stronger when there is a greater proportion of tin in the lead-free solder alloy. Copper dissolution is also affected by other variables in the thru-hole assembly process such as the pre-heat temperature of the board and the contact time between the copper plating and the flow rate of the solder fountain. The most susceptible part of the assembly is at the knee of the through hole.

The addition of nickel, silver, antimony, germanium and bismuth to the solder alloy to allow a reduction in the tin content helps to slow the copper dissolution process. However, experts say that while this process may work initially, it will not prevent copper dissolution during any reworking process.

The real solution to this dilemma is a new reworking process for thru-hole assemblies, but this still awaits development.

Feel free to contact us for more information.