Four steps for consistent joint quality in selective soldering (Part II)

In order to ensure reproducible, high-quality solder joints during selective and wave soldering, the solder bath must be subjected to continuous monitoring in order to detect any impurities that could negatively influence the soldering process at an early stage. The composition of the solder bath should also be checked and, if necessary, the solder alloy “refreshed”.

Various steps are necessary for this, which we present in parts I and II:

3. checking of additives or alloying additions

Apart from established standard alloys, some manufacturers have developed or modified their own brazing alloys, which have been mixed with additional elements. These additives are intended to give the user certain advantages in the brazing process or to avoid its negative effects. Effects such as oxidation, alloying properties, affinity, etc. cannot usually be avoided or prevented, but certain additives in the ppm range do produce short-term positive effects on various physical and chemical properties of an alloy. For example, some elements delay oxidation in the solder bath, others have a positive effect on wetting and appearance, and others delay the deposition from components or PCB contacts. Most of these alloy additions in the ppm range are subject to thermal or chemical/physical reduction over time and are therefore no longer significantly effective above a certain minimum concentration. Therefore, these elements should be checked regularly in order to be able to take countermeasures in time.

4. definition of intervention limits of specific or individual elements

The basic composition of the standard alloy is not to be neglected either. Most of the standard alloys in use today are found in the lead-free range with basic compositions of tin, silver and copper. These standard alloys are usually also used in variable compositions, or pre-alloyed with other elements for special properties (prominent example: bismuth in low-melting, lead-free solders). While in the case of a standard alloy, for example, an increase of 0.5% for silver usually has no significant negative effect, this can very well mean significant negative effects for copper. Therefore, whether micro-alloyed or not, a regular solder analysis should generally be considered for a verifiable solder bath management, in order to be able to intervene in time. Especially for elements such as Ge, P, Bi, Ce, Co, which are consumed relatively quickly, a consistent solder bath management is the key to continuous quality.