Poor Solderability of Leads | Center for Advanced Life Cycle Engineering

Defect Description

The poor ability of the leads to be wetted by molted solder which results in a mechanically weak solder joint [1].

Defect Formation Process(s)

Poor solderability of leads can arise from a variety of reasons for the commonly used double layer technique in which a protective layer on top of a solderable layer, are coated on the base material of leads.

(i) Improper thickness control of the solderable layer during plating processes [2]

• Molten solder can reach unsolderable base material and dewets from surfaces if the solderable layer is too thin.

• Residual stress come with excessively thick solderable layer can cause delamination between the layer and base material of the lead

• Organic contaminants hidden in thick solderable layers volatizes during reflow and results in voids formation.

(ii) Improper thickness control of the protective layer during plating process [2]

• Small pin holes, fissures, and other breaches can compromise the protective layer if the layer is made too thin, which can cause oxidation or contamination of the underlying solderable layer

• Residual stress come with excessively thick protective layer can cause delamination between the layer and the underlying solderable layer

(iii) Extended period of exposure to ambient atmosphere can cause oxidation and corrosion of the leads [1,2].

(iv) Another cause can be deposited contaminants on the lead surface such as dust and chemicals [2].

List of Tests to Precipitate this Defect	Failure Acceleration	Likihood to Precipitate Defect (condition)	Failure Mechanism(s)
Thermal Shock	• Cyclic thermal mechanical stress accelerates fatigue of weak contacts • Thermal mechanical stress can cause fracture of weak contacts	✔	Thermal Fatigue Thermal Mechanical Overstress
Random Vibration (RS/ED)	• Random Vibration accelerates fatigue of weak contacts • Random Vibration can cause fracture of weak contacts	✔/✇ (Defect orientation is sensitive to the vibration axes)	Mechanical Fatigue Mechanical Overstress
Combined Environment	• Combination of Thermal Shock and Random Vibration	✔	Combination of Thermal Shock and Random Vibration
Bend Test	• Bending can cause fracture of the solder joint at the cracks where local stress concentration is high	✇ (Defect at a location with significant strain due to bending)	Mechanical Overstress
Temperature, Humidity, Bias	• Humidity can accelerate corrosion	✇ (Poor solderability is due to defective protective layers or deposition of contaminants)	Corrosion

[3]

References

[1] JEDEC, Solderability Tests for Component Leads, Terminations, Lugs, Terminals and Wires, J-STD-002D, 2011.

[2] Vianco P. T., “An Overview of Surface Finishes and Their Roles in Printed Circuit Board Solderability and Solder Joint Performance”, Circuit World, vol. 25, no. 1, pp. 6-24, 1999.

Permission for pictures

[3] Poor Lead Solderability and Wetting. Retrieved August 8, 2016, from http://www.epectec.com/pcb/wave-soldering-defects/poor-lead-solderability-and-wetting.html

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