SOLDER-JOINT INTEGRITY BASIC FAILURE PROCESSES TUTORIALS

Solder-Joint Integrity

Solder-joint integrity can be affected by the intrinsic nature of the solder alloy, the substrates in relation to the solder alloy, the joint design or structure, the joint-making process, and the external environment to which the solder joint is exposed.

Therefore, to assure the integrity of a solder joint, a step-by-step evaluation of the following items is warranted:

■ Suitability of solder alloy for mechanical properties

■ Suitability of solder alloy for substrate compatibility

■ Adequacy of solder wetting on substrates

■ Design of joint configuration in shape, thickness, and fillet area

■ Optimal reflow method and reflow process in terms of temperature, heating time, and cooling rate

■ Conditions of storage in relation to the aging effect on the solder joint

■ Conditions of actual service in terms of upper temperature, lower temperature, temperature cycling, vibration, and other mechanical stress

■ Performance requirements under the conditions of actual service

■ Design of viable accelerated testing conditions that correlate with actual service conditions

Basic failure processes

In the real world, solder-joint failure often occurs in complex mechanisms involving the interaction of more than one basic failure process. Although creepfatigue is considered to be a prevalent mechanism leading to the eventual solder joint failure, separate test schemes in creep and fatigue are often conducted to facilitate data interpretation and an understanding of the material behavior.

The basic processes or factors that are believed to contribute to solder failure during service are as follows:

■ Inferior or inadequate mechanical strengths

■ Creep

■ Mechanical fatigue

■ Thermal fatigue

■ Intrinsic thermal expansion anisotropy

■ Corrosion-enhanced fatigue

■ Intermetallic compound formation

■ Detrimental microstructure development

■ Voids

■ Electromigration

■ Leaching