Book Excerpt: The Printed Circuit Designer’s Guide to... Thermal Management with Insulated Metal Substrates, Chapter 2

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Chapter 2: Solder Cracks
Historically, engineering teams using some types of insulated metal substrates in high-performance LED applications have noted the formation of solder cracks after prolonged thermal cycles. Solder cracks, as the name suggests, are discontinuities in the solder joint, usually caused by excessive mechanical stress. This stress typically results from a mismatch between the coefficients of thermal expansion (CTE) of each of the joined components. Through repeated thermal cycling, the tendency of the joined parts to expand and contract at different rates eventually overcomes the solder’s ability to resist that differential displacement. A crack then forms in the solder. Crack formation like this can be aided by soldering defects such as poor wetting at either surface or inclusion of impurities within in the solder.

A solder crack impairs both the mechanical integrity of the joint and the electrical continuity. The mechanical weakness may not be detected, especially if there are several such connections and others are unaffected. The electrical discontinuity may be partial, manifested as increased ohmic resistance if the separated surfaces remain in contact, or may cause an intermittent open circuit. Each can cause defective functioning of the circuit or system.

Figure 2.1 shows possible root causes of such solder cracks. Although this list is not exhaustive, any combination of these effects can contribute towards conditions that cause solder cracks to occur.

As a designer and manufacturer of laminate, Ventec can directly influence multiple aspects relating to materials selection and use, including:

• Minimizing percentage of copper on circuit side
• Use Super HTE (High-Temperature Elongation) copper foil with lower tensile strength
• Lower Tg dielectric working in elastic deformation zone
• Lower CTE of aluminum baseplate (CTE is 19 ppm/K)
• Copper baseplate
• Choosing high-performing dielectric

In addition, the effect each crack or void in the solder joint has on the thermal path of the system must also be considered. A representative approximation is that a crack affecting 50% of the solder pad area will double the thermal impedance.

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