Does anyone have any failure information on IRC, CHP2-100-R500F (5 OHM, 2W, 1%) resistors. We have experienced two failures on the same circuit board where the metalization on the component separated from the component causing an open circuit. There is a 1/2W fuse in-line with this component. The circuit board was coated with a urethane conformal coating.
Question - Is there some chemical reaction that could remove the metalization on this type of component?
Thanks in advance for your help
Gary Bremer Manufacturing Engineer Rogerson-Kratos 626-449-3090 ext. 219
These components are 2W resistors that only see 1/2W at max. during normal use. I have contacted IRC still wqaiting for a response. The resistors were tested as good. But the circuit was open as a result of the separation of the Sn 60/40 solder plate to the Nickel undercoating. Essentially, the metal coating of the resistor ends mechanically separated.
The coating is pliable and is air cured for 24 hours. The potting compound goes into the top assemnbly in 3 diferent stages. The circuit boards are coated using urethane type conformal coating. The unit is also placed into an ESS chamber -40� to +55� Celsius.
Observations are: * Center most lead of the fuse on the top right of the assembly looks like it has excellent solder flow, but this is PTH. From the amount of solder, it must have been wave or hand soldered. * Center most lead of the fuse on the bottom right of the assembly looks like it has very good solder flow. From the amount of solder, it must have been wave or hand soldered. * Neither MELF remaining on the board looks like it has acceptable solder flow. * Terminations of the MELF remaining on the board have the same dull appearance as removed MELF. * None of the pads on the board where the MELF were removed look like they had acceptable solder flow. * Solder flow on a fairly large SMT passive, just below the fuse at the to center of the image, shows good solder flow. Although, it certainly cannot be criticized for having too solder. * Some of the pads on the board where the MELF were removed look as if the potting compound was between the solder on the pads and the component terminations. [Although this could be an artifact of the lighting for the picture.] * Layout of the pad for the MELF do not conform to IRC recommendations. * Layout of the board indicates ample use of grounds plains and some thermal relief of pads to provide for proper wetting of PTH components. * Terminations on the removed MELF look poorly wetted with no sign of reflowed solder paste. * Little or no metal bonding occurred between the solder and the base metal of the removed MELF. * Solder on the end caps of the removed MELF looks like it was peeled off of the end cap. * Termination on one removed MELF has solder flow, but it appears dewetted.
Conclusion: Components were poorly fabricated.
Alternatives considered are: * Poor reflow solder process control. That the solder reflowed well on a SMT passive near one of the removed MELF indicates that the reflow recipe was at least �pretty close�. * Conformal coating affected solder connection. That the PTH connections remain unaffected indicates that this is not the source of the problem. * Potting affected solder connection. That the PTH connections remain unaffected indicates that this is not the source of the problem.