PHOENIX -- A new white paper by Suntron purports to solve the problem of tombstoning in 0402 components.

The Low Mass Solution to 0402 Tombstoning, authored by product engineer Eric Reno, details the engineering findings surrounding an age-old issue. Tombstoning occurs when a part is pulled up on one side, assuming a vertical orientation that looks like a graveyard headstone.

In his paper, Reno says attempts to resolve tombstoning have overlooked one aspect: the physical component. "Although the physical component may be in compliance with EIA 0402, the physical characteristics of the component must be understood and ruled out," he writes.

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 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.

To view the white paper, click here.

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