Tombstone Chip Caps Revisited

John Maxwell, Chris England
Johanson Dielectrics, Inc.
Sylmar, CA, USA

Drawbridges or tombstone assembly defects for chip components has not been a topic of discussion for some years but these defects are on the increase as low cost vendors are providing parts into the electronic component supply chain. Land pattern design standards, improved solder paste materials and improved reflow soldering systems have all but banished tombstone type of defects from electronic assembly.

Unfortunately it has raised its ugly head with a new source of these old defects surfacing during ongoing competitor component evaluation of low cost Asian manufacturers of MLC capacitors. Sudden out gassing or venting of trapped nickel plating solutions has been observed in a number of BME (Base Metal Electrode) capacitors resulting in a new source of drawbridge or tombstone solder joint defects in chips as small as 0402 and as large as 1825.

Perhaps it is now time to add a test technique to evaluate plating sensitivity levels (PSL) when these chip components are reflow soldered like MSL1,2(Moisture Sensitivity Levels) ratings for molded components to minimize popcorn damage due to steam formation within the package. Unfortunately or fortunately depending on ones point of view there can only be a pass fail system vs. different levels found in MSL ratings for molded IC packages.

Component vendors should have active internal termination evaluation test methods that simulate end customer solder conditions to insure that boiling plating solutions do not rupture termination plated layers ejecting corrosive plating salts onto an assembly. The splatter is a corrosion problem and cause tombstone defects, which is the detected symptom.

The technique shown in the paper is just but one procedure that a vendor can use to determine termination density and suitability for all customer applications. At the same time customers need to validate that new low cost vendors used by manufacturing sites in Asia are using parts that do not present reliability or rework hazard due to this defect. Most of these defects were observed in BME capacitors with nickel electrodes and copper terminations during competitive part evaluation.

Bottom line, know thy vendor and ask, how they evaluate termination and plating integrity. Qualify all component vendors in the supply chain and do not change without evaluating a number of factors including termination splatter. What is in the MLC capacitor vendors control plan, what test techniques are used and what is the sampling frequency to insure that splatter does not occur on electronic assemblies?