Transistor Heat Sink Fatigue

Some of the older PHI electronic units have a type of heat sink with a built-in spring tab (shown in the picture below) that forces the back of a transistor into the heat sink.  Over time, the spring tab can lose tension (most likely due to heat induced metal fatigue) and then the transistor no longer connects to the heat sink, eventually resulting in the transistor failing due to overheating.

For units such as the 32-100 Electron Multiplier Supply, overheated transistors are often the cause of multiplier voltage output problems. 

For this blog post we will look at a 32-100 electron multiplier supply with no output on the CMA high voltage output. The problem was isolated to a bad TIP120 transistor which shorted out and melted because the back of the transistor separated from the heat sink over time.

In addition to replacing the TIP120 transistor, we also modified the heat sink to ensure a good contact with the transistor. 

To modify the heat sink, you need to first remove the defective transistor and then remove the heat sink.  You will need a hot soldering iron as the heat sink has enough mass that it will drain away some of the heat from the soldering iron.   You can use a solder sucker or some solder braid to remove the solder from the heat sink contacts.

Once the heat sink as been removed, break off the spring tab.

Next, drill a small hole in the back of the heat sink where the indent is located. We used a 9/64″ drill bit since we needed to clear a 6-32 screw and lock nut.

Put some heat sink compound or conductive tape on the back of the replacement transistor.  This is necessary to ensure good thermal transfer from the transistor to the heat sink.

Use a screw and lock nut to attach the transistor to the heat sink.  Make sure that the transistor is centered in the heat sink.   In this case we also added another small heat sink to the back of the original heat sink to add some additional cooling for the transistor.

Next, insert the transistor leads into the holes on the board and insert the heat sink into the larger holes in the board.   Solder the heat sink and the transistor leads.  Cut the excess leads from the transistor and remove any excess flux from the board.

Now that we have replaced the transistor and improved the transistor to heat sink contact, the 32-100 should perform well for many years.

Since we were replacing the one defective TIP120 transistor, we also replaced the one for the SED supply as well (and modified its heat sink) as a preventive measure.  In this case we could not add the extra small heat sink due to a tight clearance to the nearby transformer. Even so, the improved contact to the heat sink will provide improved heat transfer from the transistor and result in improved reliability.

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