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Key info from the MSL Lab

Posted: 23 Mar 2009     Print Version  Bookmark and Share

Keywords:Plastic encapsulated microcircuits  Moisture Sensitivity Level  reflow 

2. Moisture clings to surfaces
Moisture that is absorbed through the mould compound eventually is collected on an internal surface. Since the largest internal surface in a PEM is the die paddle, this is where moisture is most likely to be present. In some PEMs, moisture is also absorbed by the die attach material. Moisture in either location is likely to flash into steam during reflow and cause damage. If the die attach is the culprit, changing the die attach material may help.

Figure 2: The vast majority of popcorn cracks travel downward (B), and may never be noticed during assembly. Those that travel upward (A) are very likely to break wires or bonds and cause immediate electrical failures.

3. Most popcorn cracks go downward
The most frequent internal damage that Melville sees is some type of popcorn crack. Popcorn cracks typically originate near the die attach (or the die attach to die paddle interface), and then travel either upwards or downwards (Figure 2). The vast majority of popcorn cracks that Melville sees travel downward towards the bottom of the package. Traveling downward, these cracks avoid wires and thus are unlikely to cause electrical failure that would be caught by testing. The downward popcorn cracks often exit the bottom of the package, though, and would therefore never be noticed during assembly. The open crack provides a direct pathway for moisture and contaminants to find their way into the centre of the package, where corrosion can later cause an electrical failure.

Whether a popcorn crack travels upward or downward depends on the exact structure of the package, Melville says. In some plastic BGA packages, the same phenomenon results in a horizontal popcorn crack—or, more properly, a popcorn delamination—between the mould compound and the substrate.

4. Cracks begin at the die paddle
Melville has noticed that a crack that begins in the die attach is most likely to originate at the interface between the die attach material and the die paddle. The reason: die attach material typically adheres more strongly to the die than it does to the die paddle.

5. Tiny die on large paddles often fail
Cracks are especially likely to occur, Melville notes, if the die paddle is relatively large and the die is especially small. If the die paddle and the die have similar areas, it seems less likely that sufficient moisture will gather to allow a crack to occur. The worst case is a tiny die on a very large paddle, where cracks or delaminations sometimes pull the die away from the paddle. Some PEM designs try to solve this problem by placing grooves or dimples in the die paddle to increase adhesion.

- Tom Adams
Consultant
Sonoscan Inc.


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