Thick Film Resistor Terminations - Eliminating Long Term Reliability Issues
Considerable effort may go into designing a thick film resistor product to survive demanding environmental conditions. However, failure to consider the resistor terminations to the system board can lead to reduced performance and/or premature device failure.
There are two main resistor component mounting methods, surface mount and through hole. Single surface mount resistors are generally constructed with solder pads that are soldered to the circuit board. In contrast, multi connection resistor devices such as resistor networks may have solder pads, surface mount lead frames or occasionally BGA footprints.
The three main types of leaded resistor devices are axial, TO and network type (multi lead) packages. Axial resistors have a circular cross section lead compared to TO and network resistor rectangular cross section lead profile. As in surface mount devices, the lead area is directly related to the current carrying capacity of the resistor device.
Lead materials are generally copper for axial devices. TO and resistor network devices may use copper, kovar or phosphor bronze for the leads depending on the lead frame style and the application.
A variety of lead coating materials are available including gold, platinum silver, tin and platinum palladium. Gold is a good electrical and thermal conductor with low contact resistance and excellent long term stability.
Tin is comparatively cheap but can (depending on the environment) tarnish with age and compromise the connection. Platinum silver is often used on surface mount pads as the material provides acceptable performance in most applications at relatively low cost.
A tin coated lead may be directly soldered to the system board whereas a direct gold/tin interface should be avoided. Palladium alloys can be a better choice than gold in some applications as the need to remove gold from the lead before soldering is removed. However, the costs of palladium and platinum tend to be higher than gold.
Thick film resistors may be subjected to vibration, shock, thermal cycling and/or high humidity depending on the application. The resistor package may be designed to survive extreme environmental conditions but the lead interface with both the system board can be overlooked and is often the point of component failure.
The most common cause of a compromised connection cycling temperatures that approach the solder melting temperature. This can cause crystallisation of the solder joint and eventual failure.
Poor handling practice can also unduly stress or damage the resistor device leading to premature failure. This is a particular problem with higher power devices with larger leads. The larger leads can make the lead forming activity and/or attachment process more difficult and robust handling may damage the resistor device.
It is important the demands of the application are assessed before choosing a resistor device. The lead material, lead finish and connection method to the system board should be chosen to prevent any long term reliability issues. Where a standard component is not available to match the demands of a particular application a specialist resistor manufacturer may provide a custom solution.