fltcoils
Member
I'm posting this here FYI, in part because I think some would appreciate the metallurgy aspect, and in part because this problem affects many items using a Vacuum Fluorescent Display.
The MPG readout on my used Chevy Venture never worked. It is located on the headliner between the visors. These can cost $300 and up to replace.
One day, after a rain, the typical water leak from the sliding door joint had again dribbled down into the readout module (Driver Information Display). I could see water in the display window, there was enough collected to have filled the thing. But...when I started the car the display was working for the 1st time. I immediately turned off the car and drained it of water. It no longer functioned.
Then I began research. I learned how it was a Futaba VFD or Vacuum Fluorescent Display unit. These are similar to triod vacuum tubes but operate on only 30-50 volts plate supply. In some units (Sony Betamax) the discrete intergrated circuit cans used to create the plate supply malfunction (the caps dry out). This was not the case with this display that I had. Didn't fit the symptoms.
The solution was similar to that found for similar modules on other brands of vehicles and consumer applieances
There are four large surface mount resistors by the display module. I believe they are part of the plate or cathode bias circuit. One of these apparently runs hot. Hot enough to provoke tin migration in the solder leading to a fracture across the mounting pad face. I resoldered the resistor into place using a bit of solder and 15 Watt pencil soldering iron. All fixed.
Solder often is a mixture of tin and lead. The tin is more strongly attracted to the copper in the circuit board than the lead is attracted. When hot, when within 10% of melting temperature (resistors designed to run hot), the tin near the circuit board face will move. Having moved there is stress in the solder at the locations where it left. These holes create high stress in the solder structure, this leads to a fracture staight across the mounting face of the resistor as smooth and complete as if a knife cut the resistor loose. This only happens in designs which allow parts to heat to values close to the melting point of the solder. This rarely would happen in the past before surface mounted components. The cure would be higher temperature solder, cooler running parts, avoiding flat straight mounting interfaces, etc. Reheating the joint sets it right for a while.
I don't know if this will help anyone else fix things, but it was interesting researching these things and satisfying to achieve a repair.
The MPG readout on my used Chevy Venture never worked. It is located on the headliner between the visors. These can cost $300 and up to replace.
One day, after a rain, the typical water leak from the sliding door joint had again dribbled down into the readout module (Driver Information Display). I could see water in the display window, there was enough collected to have filled the thing. But...when I started the car the display was working for the 1st time. I immediately turned off the car and drained it of water. It no longer functioned.
Then I began research. I learned how it was a Futaba VFD or Vacuum Fluorescent Display unit. These are similar to triod vacuum tubes but operate on only 30-50 volts plate supply. In some units (Sony Betamax) the discrete intergrated circuit cans used to create the plate supply malfunction (the caps dry out). This was not the case with this display that I had. Didn't fit the symptoms.
The solution was similar to that found for similar modules on other brands of vehicles and consumer applieances
There are four large surface mount resistors by the display module. I believe they are part of the plate or cathode bias circuit. One of these apparently runs hot. Hot enough to provoke tin migration in the solder leading to a fracture across the mounting pad face. I resoldered the resistor into place using a bit of solder and 15 Watt pencil soldering iron. All fixed.
Solder often is a mixture of tin and lead. The tin is more strongly attracted to the copper in the circuit board than the lead is attracted. When hot, when within 10% of melting temperature (resistors designed to run hot), the tin near the circuit board face will move. Having moved there is stress in the solder at the locations where it left. These holes create high stress in the solder structure, this leads to a fracture staight across the mounting face of the resistor as smooth and complete as if a knife cut the resistor loose. This only happens in designs which allow parts to heat to values close to the melting point of the solder. This rarely would happen in the past before surface mounted components. The cure would be higher temperature solder, cooler running parts, avoiding flat straight mounting interfaces, etc. Reheating the joint sets it right for a while.
I don't know if this will help anyone else fix things, but it was interesting researching these things and satisfying to achieve a repair.
