The Low Mass Solution to 0402 Tombstoning
Tombstoning occurs when a part is pulled up on one side, assuming a vertical orientation that looks like a graveyard headstone. Suntron performed a dimensional evaluation on seven manufacturers of capacitors and six manufacturers of resistors commonly used by its customers. The analysis, which looked at the component body and its terminations, revealed was that 86% of the capacitor manufacturers and 50% of the resistor manufacturers had different body and termination dimensions and tolerances. (By comparison, a similar evaluation of the same component manufacturers' 0201 and 0603 package types showed identical parts.)
Reno says these variations in the components must be accounted for in the pad geometry, or else tombstoning may occur. He also recommends treating each pad as a group, and ensuring the copper density of each pad is equal (or very close), meaning both pads achieve the same temperature and liquidus at the same time. Also, Reno says, both pads should achieve solder flow to exposed copper at the same time, and be equal in solder volume necessary to control capillary action.
Reno recommends a specific pad geometry for 0402a, reduces the soldermask clearance to 0.002" (from 0.005"), and suggests a connecting trace between pad and plane (or very wide trace) to be equal as the sister pad.
Author: Eric Reno, product engineer II