Whisker (metallurgy)
Whiskers (English for Whisker or whiskers ), also hair crystals are needle-shaped single crystals of a few microns in diameter and up to several hundred micrometers in length, which grow out of electrically or by pyrolysis deposited layers. In the range of solder whiskers are known having a length of several millimeters.
Whiskerneigung of materials
Some materials have an increased tendency to whisker. These include, for example, antimony, cadmium, indium, zinc and tin.
Since whiskers are single crystals, they have a very homogeneous structure, almost free of errors. With them still can, for example screw dislocations occur. Whiskers have much higher strength than polycrystalline structure. Ceramic cutting tool materials are mixed for example with SiC whiskers to increase the wear resistance.
When whisker growth is a dynamic process with numerous influence. The longitudinal growth of the whiskers can be up to 3 mm per year.
Whiskers in electronic assemblies
Whisker growth
Whiskers formed during assemblies partially only after years of operation. Here, the onset risk of the outside can not be measured. The whisker has only an effect if there is an electrical connection between two electrical networks.
When it comes to whisker transport of atoms. Whisker growth occurs most frequently at intermediate temperatures. At low temperatures, the mobility of atoms is not as great. For larger temperatures, stresses in the material can break down more easily. Tin (part of the solder material ), the average temperature range between room temperature and 70 ° C / 80 ° C.
Furthermore, the tin must be considered in the whisker at a solder alloy. When the tin content of the alloy is less than 70 percent, the effect of the whisker does not occur.
Furthermore, the whisker growth occurs more on components or printed circuit boards that are subject to mechanical stress.
In the whisker must be made between electrodeposited layers and melted layers. The melted layers generally have a lower probability to whisker.
Consequences of whisker
The current capacity of the whiskers is in the mA range and can be up to 10 mA. For larger currents, the whisker burn, although often through again, until then the current flowed but may have led to component damage or malfunction.
Whiskers arise particularly easy for modules that have been processed with lead-free tin solders, and can cause short circuits galvanically produced printed circuit boards or between components. In safety- critical applications of electronics, such as ABS or ESP systems in motor vehicles continue to lead containing solders are used, since so far only by the whisker formation can be prevented by a lack of long-term stability. With the introduction of the new ELV, which prohibits inter alia, the use of lead-containing materials, lead-free soldering process for the entire car electronics are mandatory.
Suppression of whisker growth
The whisker growth can be substantially suppressed by the application of intermediate layers. As a frequent intermediate layer in this case is nickel application. So that this intermediate layer is also effective, it must have a minimum thickness of 3 microns. As another materials for the intermediate layers of gold and silver can be used.
Generally tend electroplated layers reinforced to whisker. By the use of hot- applied layers, the risk of whisker formation can be minimized. Furthermore, it is an improvement, when electroplated layers are melted subsequently.
Furthermore, the whisker can be minimized by changing the voltage applied to the circuit boards and assemblies mechanical stresses are minimized. This includes mechanical forces to clamp and screw- in modules.
By the use of lead-containing solder materials in place of the lead-free solder materials, the risk of whisker growth can be minimized. When lead-containing solder the tin is frequently in the range of 60 percent, while the proportion of tin is lead-free solders for well over 90 percent.
A coating of the printed circuit board with a protective coating (conformal coating ) intended to protect against moisture and dirt can slow the growth of whiskers, but not prevent it.