Imaging experts to the rescue
The researchers chose to contact colleagues at DTU's 3D Imaging Center, where advanced X-ray methods can be used to ‘look into’ materials and components. The X-rays of the diode confirmed that some material inside the diode had migrated and created the short circuit. The images also gave the researchers an idea of where in the diode the problem had occurred. This enabled the researchers to make a precise cross-cut of the diode, which meant that they cut the diode crosswise and then cut through until they reached the exact location of the migrated material.
“Even though our supplier was very active in finding the fault, they had not thought about making a cross-cut. Here, DTU researchers were able to contribute with a method and some subsequent results that we would not otherwise have been presented with,” says John Jacobsen.
With the cross-cut, DTU researchers discovered that silver had migrated inside the diode. Silver is a standard material in many electronic components, and here it was applied to the surface of the diode, so enable soldering on the surface. Silver is also included in the material used for soldering.
“A 10-micrometre-thick layer of silver was found at the location of the diode’s electrical breakdown. That’s a lot in one diode. It was clear that a violent electrochemical migration had occurred,” says John Jacobsen.
With DTU’s help, Grundfos was now able to document - both with images and physically - that the fault in the diode had occurred as a result of silver having migrated and created a short circuit.
“This has enabled us to have an open dialogue with the supplier about the error that may occur, and together we can now find another diode solution where silver is not included as a material. In this way, we avoid the same error occurring in the future,” says John Jacobsen, who adds:
“The collaboration with CreCon means that I have highly competent partners at DTU who are independent and with whom I can discuss and analyse things. It reassures me that I can get to the bottom of a problem, which is particularly important if you suddenly have a supplier who is not very cooperative.”
Improper use of pumps
But why did the silver migrate in the first place? This requires, among other things, moisture, but how was moisture able to penetrate a diode being fully enclosed in epoxy?
"We found that the pumps in the office buildings had been installed just before the pandemic lockdowns. The people responsible for the buildings tested the cooling system and forced cold water through the pipes without the pumps being turned on. When you draw cold water through, everything will cool inside the pump and condensation will occur in the electronics. This will usually not be a problem when the pumps are turned on, because then the electronics will generate a little heat, which will cause the condensation to evaporate. But when the pumps are turned off, condensation is allowed to accumulate, and this has led to soaked electronics that, even after it is turned on, cannot compensate for weeks, perhaps months, of inappropriate use,” explains John Jacobsen.
In addition to choosing a new type of diode - one without silver - the clarification work has also led Grundfos to recommend to their users that the pumps should always be switched on when water is drawn through the cooling systems. This prevents condensation from accumulating inside the pump electronics.