Help, My Semiconductors Are Desoldering Themselves!

Semiconductors make the world go round, and they quietly operate in the background of many systems providing some of the most important electrical functions. Many semiconductors can have very high operating temperatures before they start to burn out or fail. The high operating temperatures of some discrete semiconductors can be so large that components begin to excessively heat up a PCB. When this happens, and heat becomes trapped around the component, you might see components start to desolder themselves before they burn out and fail.

If you have semiconductors getting so hot that they desolder themselves, what can you do about this problem? Furthermore, what is the root cause that leads to this mode of failure? Semiconductors self-desoldering actually has some simple causes, particularly when we look at the broad range of discrete semiconductors. Don't expect this to happen with integrated circuits due to their somewhat lower failure temperatures, but discrete semiconductors can experience this problem.

Root Causes of Excessive Temperature in Discrete Semiconductors

Discrete semiconductors are a broad class of components that includes just about every semiconductor you'll find internally in an integrated circuit. This includes transistors, diodes, varactors, triacs, and thyristors. Depending on how the components are operated, they can experience excessive heat that leads to failure. Some components experience other failure modes before they desolder, so desoldering will not be noticed in all situations.

Typically, there are two broad causes that may lead to failure of a discrete semiconductor:

• Running the device at excessively high current

• Failure of the device to fully modulate conduction

The second cause actually leads to the first cause, but it arises in AC circuits rather than DC circuits. The end result is the same in both cases: either the component burns up or it desolders itself and falls off the PCB.

Desoldered MOSFETs (source: EEVblog)

With this failure mechanism identified, let's look at two prototypical examples of components that can fail via desoldering: MOSFETs and diodes.

Diode Desoldering

Diode desoldering in the DC case is very simple; the device is run at excessively high DC currents and the channel resistance generates too much heat. As a result, the component either burns up or it experiences desoldering and falls off the PCB. This case is simple to understand but is less common. The more common instance actually arises in modulated (switching) circuits and has to do with the diode’s reverse recovery time.

When a diode is driven in forward bias, and then the bias is suddenly switched to reverse bias, the diode’s conduction state needs to switch from conductive to non-conductive. This has to do with diffusion of charge carriers into and out of the depletion region in the semiconductor. Diode reverse recovery times can vary greatly, ranging from microseconds to nanoseconds for ultra-fast switching diodes.

Fast switching diodes like the BAS316-HF can have very low reverse recovery times.

When a diode is switched into reverse bias, it needs to switch its conduction state at a rate which is similar or faster than the switching waveform driving the diode. For example, in switch mode power supplies, the driving signal is a PWM waveform, which can have a fast edge rate. If the diode fails to switch within the driving signal’s edge rate, then it will be conducting and will dissipate power. This leads the diode to heat up, and the temperature can be very high even for moderate currents.

The solution: use a faster diode with a higher current rating. The package thermal resistance might also need to be reduced. Most often, this means the diode will need to be physically much larger.

Desoldering Failure With MOSFETs

MOSFETs also have modulated conduction using a driving waveform applied to the gate. The driving waveform modulates the conduction on and off, or between low and high drain Source resistance values.

Similar to diode reverse recovery, MOSFETs have a rise time and fall time specification that states how fast the component can switch between conducting and insulating states. The body diode in a MOSFETs also has a reverse recovery time which tends to be similar to the turn-on/turn-off time for the channel in the MOSFET.

Example turn-on/turn-off specs for an IRF4905 MOSFET

When both times are much longer than the switching waveform, the MOSFET will fail to fully modulate between its two states, and it will dissipate heat. These switching losses increase when the drain-source current increases. A related problem is the strength of the gate driver, which might not provide enough current to supply total gate charge to the FET to fully turn it on, giving excessive channel resistance. The result is excessive switching losses that create a risk of overheating.

MOSFETs can also fail under DC for the same reason as diodes; they can be driven at excessive current. When driven at excessively high voltage, current, and or temperature, the MOSFET can fail to a short circuit. Once the device fails to a short circuit, an injected current will continue to dissipate heat and drive the MOSFET to thermal runaway. The eventual result is complete burnout of the MOSFET or desoldering (or both).

The solution: eliminate mismatch between switching time, driving waveform, and current handling. In DC, current handling is much more important because the driving waveform can usually be slowed down. In AC, turn on, turn off, and reverse recovery times are more important. These need to be matched up with switching frequency and switching edge rate.

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