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The problem: Diodes are being run at a load current higher than they can handle and still maintain the junction temperature within the rating. Heat is generated at the diode junction and must be removed by conduction through the lead wires into the copper track on the PCB. The heat is then radiated into the air by the lead wires and traces. The problem is the board design is such that there is not enough surface area to radiate the heat fast enough so the temperature of the lead wires and traces rises and eventual approaches or exceeds the melting temperature of the solder.
The diodes are typically rated for a maximum operating temperature of 150 °C (572 °F) which is above the melting temperature of the 60/40 solder (375 °F) typically used for this type of product. By the time the solder has reflowed the junctions are well beyond the rating for reliable operation and fail. When a diode junction fails it typically shorts which takes it “out of circuit” and reduces the diode count in the string which decrease the voltage hold-off capability of the assembly as well as increasing heating in the remaining working diodes. Once a diode or two have failed the situation becomes critical and a catastrophic failure occurs complete with arcing, smoke, fire, etc.
The Machlett Solution: Our new design uses a combination of MUCH larger trace area and diodes with a lower forward voltage drop, lower junction capacitance, and lower leakage current, all of which are heat generators. The effect of our design is a lower operating temperature due to less heat generation within the diode combined with better heat removal. We are also using the newer style SAC305 solder (lead-free) which has a melting temperature of 221 °C. These diodes come with a full manufactures quality warranty. No other diode in the industry has a warranty.