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| Field Report : Top : The Mechanism of Solder Cracking |
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The Mechanism of Solder Cracking |
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The Mechanism of Solder Cracking |
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Hirokazu Tanaka*/Yuuichi Aoki*/Shigeharu Yamamoto*
Soldered jointings are indispensable to high-density surface mounting of electronic equipment. Maintaining the reliability of soldered jointings is going to become a major problem that must be overcome to accurately mount miniaturized electronic parts. These soldered jointings deteriorate with exposure to long-term heat and mechanical stress, causing cracking which leads to part failure. In this paper we shall report on reliability testing to confirm the mechanism of solder cracking due to heat stress. |
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| Many problems exist in relation to soldering reliability, but in this report we shall deal with analyzing the solder cracking mechanism caused by heat cycles. |
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| 2. Solder and Solder Cracking |
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Improving reliability has become vital due to the increase in the number of soldered jointings and recent high-density surface mounting. When observing a cross section of solder cracking that generally occurs in the field, we can confirm that the particles of each phase of Sn (tin) and Pb (lead) have roughened when compared to the initial soldered jointings. This roughening phenomenon can be confirmed by heating solder to high temperatures and by storing long-term at room temperature.1) Furthermore, in cracking occurring in a jointing interface with Cu (copper), only Pb leaves any significant residue on the jointing interface. This is because at high temperatures the dispersing of Sn toward Cu is accelerated, and only Pb remains on the jointing surface, causing a degradation of jointing strength. Considering these factors, we performed the High Temperature Storage Test, the Temperature Cycle Test (air chamber method), and the Thermal Shock Test (liquid bath method) to investigate the effects of high temperatures, and the effects of heat stress due to the temperature cycle. We would like to report our confirmation of the solder cracking mechanism. According to the results of those tests, the crystallization of solder is changed by heat and stress, resulting in the degradation of mechanical characteristics.
Solder is an alloy of Sn (tin) and Pb (lead). This combination forms a eutectic alloy particularly well-suited to jointing due to such characteristics as viscosity, flowability, and melting point. When solder is cooled from the liquid phase and reaches crystallization point, it crystallizes in two solid phases, the Pb-rich α phase, and the Sn-rich βphase.2) Immediately after the jointing solidifies, these two phases are uniformly distributed as small particles. When the jointing is with Cu, such as a printed circuit board pattern, the Sn in the solder is dispersed (intergranular dispersion) within the Cu granular boundary, and forms a jointing by making an intermetallic compound. This solder jointing deteriorates due to long-term heat and mechanical stress, resulting in such phenomena as solder cracking. (Photo. 1, a and b). To confirm this failure mechanism, we performed reliability testing on the effects of heat stress. |
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Solder cracking
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(a) Solder cracking sites(100X)
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(b) SEM photo of cracking cross section(1000X) |
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| 3. Reliability Testing Results |
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| Table 1 shows testing conditions. A phenol substrate board with one paper surface was used as a specimen. Testing was begun by dipping a lead pin section with copper covered with Sn or solder plating in solder (63 Sn wt%) at 260°C for 10 seconds. After testing, solder cracking sites were impregnated with resin, cross sectioned, polished, and then observed under an SEM (Scanning Electron Microscope).
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Table 1 Test items and test conditions
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Test Items |
Test Conditions |
High Temperature Storage Test
(Using ESPEC High Temperature
Chamber model PHH-200) |
125°C, 100 hours |
150°C, 100 hours |
Temperature Cycle Test
(Using ESPEC Air-to-Air Thermal Shock
Chamber model TSA-70H) |
-65°C←→125°C,
500 cycles, 30 minutes each |
Thermal Shock Test
(Using ESPEC Liquid-to-Liquid Ther- mal
Shock Chamber model TSB-5) |
-65°C←→125°C,
500 cycles, 5 minutes each |
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| *Each test name uses EIAJ standards. |
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3-1 Results of High Temperature Storage Test
Specimens were left under high temperature for either 48 or 100 hours, then removed, cross sectioned, and observed. Results of the observation indicated that under conditions of 150°C for 48 hours the α phase showed considerable progression in roughening. Observation after 100 hours at 125°C showed that roughening progression had continued. (Photo. 2) From these results we were able to confirm that long-term exposure to high temperature promotes roughening of the α phase. |
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| Photo. 2 Changes in the α phase during the High Temperature Storage Test (500X) |
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125°C
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150°C
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Initial period
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After 48 hours
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After 100 hours
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3-2 Results of Temperature Cycle Test
Cracking occurred as in Fig. 1 in the vicinity of the substrate pin hole (section a) and near the lead pin (section b). Using the SEM to observe the section with cracking showed that roughening had occurred in the α phase, and that those particles had taken a grain-oriented configuration. (Photo. 3) Investigating the cross sectional surface of the cracks revealed surface breaking that showed signs of high stress having been applied, particularly in the surface fractured from the α phase section. In addition, in the f¿ phase some sections that have not cracked show a number of micro cracks. (Photo 4) |
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| Fig. 1 Cracking sites resulting from the Temperature Cycle Test |
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Photo. 3 Cross section of crack break (300X) |
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Photo. 4 Micro cracks in the α phase (1000X) |
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3-3 Results of the Thermal Shock Test
A few cracks can also be found in the internal section of the solder, but as seen in Fig. 2, these are at sites where the solder adjoins the lead pin (section a) or the Cu pattern (section b) and are due to separation of the interface. (Photo. 5) In addition, this intermetallic compound alloy has become thickened, and we can assume that the dispersion of Sn has progressed. Unlike the Temperature Cycle Test, no micro cracking occurred. |
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| Fig. 2 Checking sites due to the Thermal Shock Test |
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Photo. 5 Cross section of interface peeling (200X) |
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