Component Cracking

Cracking of multilayer ceramic capacitors has been attributed to excessive heating rates during reflow. This may be somewhat mythical and what little cracking is still observed usually tends to be due to defects in the manufacturing process of the capacitor. Cracking of plastic-molded packages, such as QFPs and BGAs, sometimes called the popcorn effect is sometimes due to absorbed moisture in the thermoset plastic body but may also be due to a flaw in the design and/or manufacture of the component.

Process and design-related causes:

• Improper fabrication of the component

• Improper design of the component

• Improper packaging of component during shipping

• Improper storage of components at incoming and on the manufacturing floor resulting in excessive absorbed moisture

Reflow-related causes:

• Preheat portion of profile and rate of heating too aggressive (should not exceed 4K/sec. per 20 sec. interval.

Dull solder joints

Dull (non-shiny) solder joints are usually the effect of coarse grain structure in the solid solder joint (though there can be other causes). The more slowly a joint cools, the coarser the grain growth and, conversely, the faster the joint cools, the finer the grain growth and the shinier the joint. Dull or grainy joints are not a big concern – the grain structure will grow in a joint over time anyway. But having a dull joint is giving this eventuality a “head start”. However, under no circumstances, should a joint be touched up by applying a soldering iron (or other means) simply for the cosmetics of a shinier joint. The damage you may be causing by inducing accelerated intermetallic growth will far offset any gains in joint strength through a finer grain structure.

Process and design-related causes:

• Impurity in the component plating

• Impurity in the circuit board finish

• Bad or expired solder paste

Reflow-related causes:

• Cooling too slow inducing coarse grain structure and a dull finish. Accelerate cooling by use of cooling modules. It is recommended that a cooling rate of 4K/sec. over any 20 sec. Interval is not exceeded.

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