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An analysis of solder cracking mechanisms
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Hiroko Inoki* Hirokazu Tanaka* Yuuichi Aoki* Shigeharu Yamamoto*
High-density mounting of circuit parts on electronic equipment makes the reliability of solder joints more crucial than ever. To grasp trends and improve reliability in solder joints, studies must be undertaken on printed circuit boards, mounted parts, and mounting technology. Solder cracking is directly involved in equipment failure, and controlling its occurrence has become a major problem. This report looks at one cause of solder cracking -the roughening of the solder grain boundary field- and verifies the relation of the problem to both thermal and mechanical stress. The results indicate that the solder grain boundary experiences roughening from both types of stress, although thermal stress exerts the greater influence. Degradation of solder strength due to roughening was also confirmed.
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Solder is used to join many of the circuit parts in modern equipment, and maintaining the reliability of solder joints is crucial to maintaining the reliability of electronic equipment. However, use of miniaturized electronic equipment in recent years has brought the equipment into contact with a wide variety of environmental conditions leading to a wide range of external stress, which can lead to solder cracking. Because of this, this report will detail the following studies on the relationship between stress and the roughening of the grain boundary that leads to solder cracking:
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An investigation of the solder load and stretching caused when mechanical stress is applied to solder joints, |
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An investigation of the changes caused in the solder grain boundary when load fatigue is applied to solder joints, and |
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An investigation of the changes in the solder grain boundary caused by thermal stress, as well as how changes in the grain boundary affected the strength of solder.
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| 2.Mechanisms of solder cracking |
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| Solder cracking can occur either internally within the solder or in the area where the solder joins to another surface. Both types of cracking have been confirmed to commonly occur in the field, and are a major cause of eletronic equipment failure. (Photo 1, Fig. 1)
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Internal solder cracking is caused by roughening of the α phase (Pb rich) of the solder structure. This occurs when the α phase roughening is promoted by thermal energy of temperature cycles. When stress is applied, cracks are thought to grow at the grain boundary surface within the grain boundary of the α phase and the β phase (Sn rich).2), 3), 5), 6), 7) (Fig.1 section (1), Fig. 2)
Solder joint surface cracking occurs primarily in the vicinity of layer roughening at the solder joint surface of the intermetallic compound. (Fig. 1 section (2))
When the metallic layer of the intermetallic compound grows and thickens due to high temperature, fine cracks appear in the solder joint layer, making it less resistant to shock, and giving the solder a tendency to peel.4) However, it has also been reported that in actual use the intermetallic compound does not cause cracking.5)
Reported countermeasures for solder cracking include such means as the following.
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By adding Ag, In, and Sb to the solder, the additive elements (Ag, In, and Sb) diffuse throughout the grain boundary of the Sn-rich phase and the Pb-rich phase, and the resulting intermetallic compounds (In-Sb and Ag-In) control the diffusion of Sn.6) |
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A metal such as Ni is used as a non soldering joint material with a slow rate of growth between the Sn and the intermetallic compound, suppressing roughening of the Pb-rich phase in the vicinity of the solder joint field surface, and thus improving fatigue life.6), 8)
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Photo 1 Failures
occurring in the field
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(a) Solder
internal cracking in electronic
equipment left outside for 5 years
(IC lead section)
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(b)Solder
joint surface cracking
at connector lead pin
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Fig. 1 Forms of solder cracking
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Fig. 2 Solder cracking mechanisms (solder internal cracking)
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