The exact cause of tin whiskers is still unclear (see the Useful Links area for further information on this phenomenon). Research is still being carried out on this subject. However, there are certain measures that can be implemented to help prevent whisker growth:
Plating specification – 100% tin should be used over nickel (standard plating for Harwin tin-plated connectors).
Latest research has shown that having an undercoat (usually nickel) below the tin plating has a very significant impact on the likely growth of tin whiskers. In numerous tests it has either eliminated or made insignificant any previous differences shown in matte and bright tin, studies of which were done without this undercoat plating. This is due to the elimination of compressive stress around the formation of tin-copper intermetallics (IMC).
External stress reduction – can be minimised by consideration of the component design.
Storage – Parts should be stored in dry, regulated temperatures.
Solder processing – Solder processing should avoid sudden and rapid changes of temperature where possible. This reduces the risk of compressive stress forming within the metal, due to a mismatch of the Coefficient of Thermal Expansion (CTE) of the base alloy and the plating materials.
These reports carry further information on the latest researches carried out within the industry:
- Tin Whisker Growth – Substrate Effect. Understanding CTE Mismatch and IMC Formation (no longer available on the web, please contact [email protected] for details on obtaining a copy).
Discolouration and Dewetting
Discolouration is caused by oxide formation on the plating finish. Although of significant visual impact, it is normally unlikely to affect the solderability of the product. However, Dewetting (the plating finish or solder pools into clusters on the product) is more serious, and can cause poor solder joint quality. There are steps that can be taken to minimise these incidences.
Plating specification – thin 100% tin over nickel (standard plating for Harwin tin-plated connectors).
Use of additives to minimise discolouration.
Solder Preheat – Heat should be applied at a gradual rate of no more than 1°C/sec from ambient. If possible, parts should be held at Preheat Temperature for a short period to allow all components and laminates to equalise to the same temperature.
Solder temperature – for ideal conditions, soldering should be carried out at 240°C ± 5°C. Regardless of oven temperature, component temperature should be checked using a “mole” probe which measures temperature at the pcb surface. Exposure time for the highest soldering temperatures should be kept to the minimum that will ensure a good solder joint.
Solder oven – Convection ovens give the greatest control of temperature gradients and steady temperatures, and even heating across the PCB surface. Infra-Red (IR) ovens are not recommended – process control is difficult, and surface temperatures can vary by as much as 40°C.
Extensive investigations with our plating solution suppliers has shown that discolouration is not evident until soldering temperatures exceed 300°C. The discolouration is Tin oxide, which forms in the presence of oxygen, and generally presents as a blue reflective surface effect. This effect can be avoided above 300°C by soldering under pure Nitrogen – just 0.5% Oxygen present will allow the development of oxide when molten.