Solder Challenge with “non-magnetic” Chip caps

Soldering Challenge with “non-magnetic” 0402 Caps

While doing a recent first article inspection we identified  four 0402 sized capacitors exhibit extremely poor wetting to the pad. Solder joints looked contaminated and discolored. Solder appeared to ball up on the end of the cap and did not flow down onto the board. What made this an interesting anomaly is that all surrounding parts soldered perfectly (as you can see in the photo).VJ_NM_Caps

This allowed us to rule out the oven profile, the paste, or board surface for oxidation or contamination. The capacitors  in question here are  VJ0402A1R0CNAAJ and VJ0402Y222KNAAJ. These capacitors were manufactured by Vishay Vitramon and were both in the same VJ series product line offering “non-magnetic” end terminations. Here is an interesting overview of the terminations used on Vishays MLCC capacitors. solder-reportVishay was cooperative but had very limited information about the metal composition of these parts.

My take is that they were not very confident in this product line and without saying so much, seemed aware of solderability issues. While I continued to ask questions about the alloys and any history on this part, they just kept suggesting a sample of a newer “non-magnetic” cap using a copper barrier.  In sharing this with my customer I  come to find out he didn’t select this part for its non-magnetic properties. Its not even a requirement. He selected this part doing  a simple parameter search and selection process on Digikey’s website. He entered the key parameters like many design engineers do, enter package size, capacitance value, tolerance, voltage,etc. .. the “non-magnetic” cap just happened to come up top of the list from the queries.

These capacitors we are referring to here are used in special environments where  they need to be non-magnetic (such as MRI equipment). This means there is no nickel barrier between the base material of the cap and the termination surfaces. Having no nickel barrier leaves the part susceptible to leaching, especially at Lead-Free reflow temperatures. This is what we have concluded happened with these on our build. A closer review of the datasheet identified the part is suggested for use with conductive epoxy or IR Reflow. I don’t know how well these parts perform in conductive epoxy or IR reflow processes since we didn’t evaluate that.

Through our evaluations they should not be offered for convection reflow processes as they exhibited leaching at both Lead-free and Tin/Lead reflow temperatures.