Introduction
During soldering parts that come in contact with the liquid solder will dissolve to a certain extend, in case these parts are wetted with solder.
Each material that is wettable wills at its contact boundary exchange molecules during the formation of the intermetallic layer. During this exchange of molecules parts of these intermetallics will be dissolved in the solderpot.
Intermetallic contamination
Solder may dissolve wettable metal particles to a certain level. This level depends on the metal, the solderpot temperature the tin content and the dwell time. When more metal is dissolved than can be absorbed by the liquid in a given situation, then these extra metal molecules will react with the tin, forming intermetallic crystals.
E.g. in case too much copper is depleted in the solderpot, the copper that can not be dissolved completely will form copper tin crystals like Cu6Sn5. These are needle like crystals. They are in general a few tenth's of a millimeter thick with a length of a few millimeters, up to over 5 mm. Other metals like iron show a similar behavior in case the iron content exceeds the solubility limit.
These intermetallic crystals have a higher melting point and another density than the liquid solder. When they are mixed with the solder these crystals will have an impact on the viscosity and the liquid behavior of the solder. When the solder in the solderpot is pumped around, due to its physical properties these intermetallics tend to be collected in areas in the solderpot where the solder flow speed is relatively low.
Note: In case of copper contamination in the solderpot it is quit possible that parts of the solder with a high copper content will settle in so called ‘dead corners' in the solderpot. The solder in those areas will contain relatively more copper, in the form of copper tin crystals, and will therefore have a higher melting point. This will make the solder in those areas more sluggish. On the other hand when a sample is taken from another part of the solderpot, the copper content of such a sample will be lower than the average copper content. This is what will happen in every solderpot, where outside the flow pattern 'dead areas' may occur where these intermetallic crystals will settle.
The most common reason for the sluggish solder is that most wettable metals can be dissolved in solder to a certain amount, depending on the tin content and the pot temperature. All the extra metal that is depleted in the solderpot and that can not be dissolved as free metal molecules will form in due time intermetallic crystals.
The 'dead areas' will often be found near the pump housing and in the corners of the solderpot. One can easily 'clean' the solder by removing the 'sluggish solder lumps' from these areas at regular intervals. The remaining solder in the pot, which does not contain these intermetallic crystals, is often almost as clean as the ‘original' solder. At least that solder is in general perfectly suited for making sound solderjoints.
The intermetallic particles are hard and brittle. When they are pumped around they may cause extra wear on parts which come contact with these crystals. This is another reason why the crystals, when they are formed, should be removed at regular intervals.
Exchanging the solder from the solderpot
In most cases it is not necessary or advised to exchange the complete solder content from the solderpot. If one does so, he will find that in a relative short time the contamination in the solderpot is at the same level as it was with the previous solder.
The more virgin the solder, the faster the contamination will increase. As long as the contamination level of the solder that is pumped around will not affect the solderjoint formation or the joint quality, one can use this solder. The contaminated solder that is collected in 'dead areas' will not directly take part of the solder that is pumped around, unless this contaminated solder is piled up too much and then will lose its crystals, which then will be pumped around and affect the solderjoint quality.
Since most of the contamination will collect in specific parts of the solderpot, the best way to 'clean' the solder is to remove only the most contaminated solder from these areas.