Selective solder nozzles with gold surface finish

Technical article | November 2020

Many advantages of selective soldering are based on the wettable surface of the solder nozzles in combination with the freely programmable time-path control of the solder wave, which can be used to positively influence the solder peel-off, e.g. to effectively prevent solder bridge formation.
The wettability of the nozzle surface is based on the chemical reaction of the liquid tin with the metal of the surface, forming an intermetallic phase at the interface. A disadvantage of this technology is the wear of the nozzle material due to leaching, caused by phase formation. It is therefore obvious to choose metals that leach very slowly in tin. One possibility is the coating of the nozzle body with the nickel-gold surface system known from the PCB industry. This approach was implemented by Ersa in 2005 and extensively tested in field trials at various customers. The experience with these nozzles has been consistently positive, especially the lifetime, but this surface finish has not established itself on a broad basis in the industry.

1. The gold, which protects the nickel against oxidation, dissolves immediately on first contact with the solder, while the nickel layer ensures wettability for operation. The relatively large Ni surface area should not be underestimated, as the nickel also goes into solution. If the assemblies also have a NiAu surface, a massive accumulation of these elements occurs in the solder pot. In combination with Ni- doped solders, the permissible limits of these elements are quickly exceeded.
2. The accumulation of nickel in a solder already doped with nickel leads very quickly to the saturation of the solubility of nickel in this alloy. As a result, intermetallic particles of the composition (CuNi)₆Sn₅ are formed in the melt, especially during supercooling (peel-off). These intermetallic particles develop long needles with a length of up to 10 mm. This can be seen e.g. very nicely inside a solder nozzle when it cools down slowly (see photo on the right). On the assembly, short circuits are possible if single needles or fragments break off during the soldering process and reach the solder joints with the solder.
3. Further concerns of customers about the gold surface are based on the J-STD-001 standard and here especially the point “gold removal” of components before soldering, which is by no means negligible. Although the thickness of the gold layer on the nozzle is smaller, its surface area is larger.

The decision whether to confront the personnel in the production departments with these problems is primarily up to the user. Nozzles with gold surface finish are available from Ersa on request.