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A Systems Approach to Solder Joint Fatigue in Spacecraft Electronic Packaging Differential Expansion Induced Fatigue Resulting from Temperature Cycling Is a Leading R

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A Systems Approach to Solder Joint Fatigue in Spacecraft Electronic Packaging Differential Expansion Induced Fatigue Resulting from Temperature Cycling Is a Leading R Reprinted from June 1991, Vol. 113, Journal of Electronic Packaging A Systems Approach to Solder Joint Fatigue in Spacecraft Electronic Packaging Differential expansion induced fatigue resulting from temperature cycling is a leading R. G. Ross, Jr. cause of solder joint failures in spacecraft. Achieving high reliability flight hardware Jet Propulsion Laboratory requires that each element of the fatigue issue be addressed carefully. This includes defin- California Institute of Technology ing the complete thermal-cycle environment to be experienced by the hardware, develop- Pasadena, CA 91109 ing electronic packaging concepts that are consistent with the defined environments, and validating the completed designs with a thorough qualification and acceptance test pro- gram. This paper describes a useful systems approach to solder fatigue based principally on the fundamental log-strain vs log-cycles-to-failure behavior of fatigue. This fundamen- tal behavior has been useful to integrate diverse ground test and flight operational ther- mal-cycle environments into a unified electronics design approach. Each element of the approach reflects both the mechanism physics that control solder fatigue, as well as the practical realities of the hardware build, test, delivery, and application cycle. Introduction Mechanical fatigue of electronic component solder joints is involve filling the gap between the part and the board with a an important failure mechanism that must be dealt with carefully high-thermal-conductivity heat transfer material such as alumina in any high reliability electronic packaging design. Spacecraft or copper. Figure 3 illustrates the application of copper heat electronic hardware is particularly vulnerable because the op- conductors under high-dissipation DIP packages. A historical portunity seldom exists to repair or rework the hardware after problem with this heat-sinking approach is that the DIP package launch. The one-of-a-kind build philosophy, the vacuum-ther- does not provide any lead flexibility in the normal-to-the-board mal environment of space, and the restriction to the use of only direction to accommodate the thermal expansion of the heat- Hi-Rel components, also place important constraints on the se- conduction spacer; this creates a high stress configuration that is lection of various electronic packaging approaches, and assem- particularly vulnerable to fatigue of the solder joints during tem- bly and test procedures. perature cycling. Figure 1 shows an example solder joint fatigue failure that With complex part mounting concepts such as these, achiev- occurred during ground testing of JPLs Magellan spacecraft elec- ing high-reliability long-life spacecraft electronics requires that tronics (Ross, 1989). The pictured lead is associated with a each factor in the fatigue of solder joints be understood thor- Dual Inline Package (DIP) integrated circuit mounted using a oughly and addressed systematically. Important elements in- planar surface mount to allow the opposite side of the printed wiring board (PWB) to be bonded directly to the supporting chassis heat-conduction surface. For compatibility with surface mount- ing, most integrated circuits are obtained and used at JPL in either flat-pack or DIP packages with the leads bent in the gull- wing configuration illustrated in Fig. 2. This lead configura- tion provides for a positive spacing between the part and the board and provides part-board strain relief in the plane of the board. For most parts, adequate heat transfer is provided by heat conduction down the part leads to the solder pads and through the PWB to the heatsink web. However, for high-power-dissi- pation parts, supplementary heat transfer paths must often be provided in parallel with the part leads. Such paths generally Contributed by the Electrical and Electronics Packaging Division and pre- sented at the Winter Annual Meeting, Dallas, Texas, November 25-30, 1990 of THE AMERICAN SOCIETY OF MECHANICAL ENGINEERS. Manuscript received by the Fig. 1 Cracked solder joint of DIP in spacecraft electronics caused EEP Division August, 1990; revised manuscript received February 1991. by expansion of clear Solithane in space between DIP and PWB Journal of Electronic Packaging JUNE 1991, Vol. 113 / 121 Fig. 3 DIPs mounted over 0.060" copper heat sinks Fig. 2 DIP and Flat-pack electronic part packages with leads bent for surface mounting clude careful definition of the complex fatigue environment to be experienced by the hardware, development and fabrication of electronic packaging concepts that are consistent with the de- fined requirements, validation of the completed designs with a thorough qualification and acceptance test program, and com- munication of the fatigue endurance limitations of the hardware to mission operations personnel. Each of these elements must reflect both the mechanism physics that control solder fatigue as well as the practical realities of the hardware build, test, deliv- ery, and application cycle. Solder Reliability Physics Overview Before describing the systems approach being used to control Fig. 4 Solder-joint fatigue life versus cyclic strain range (IBM data solder failure at JPL, it is useful to first review the fundamental from Wild, 1975) physics underlying the nature of solder failureparticularly the controlling parameters and their sensitivities. Considerable re- search has taken place over the last several years that has both lar structure. Because the equilibrium concentration of tin and illuminated the generic issues of mechanical failure of solder, lead in the grains is a strong function of temperature, the chemi- and highlighted the unique and complicated properties of solder cal composition of the solder grains is determined by the cooling as an engineering material (Lau and Rice, 1985, and Lau, 1991). rate during solidification and the aging time following solidifica- In general, solder is a reasonably forgiving and tolerant mate- tion. Interdiffusion between the grains over time also leads to rial with a desirably low melting point that makes it ideal for growth of the size and spacing between the grains. The size and making electrical attachment to parts that are compatible with chemical composition of the solders granular structure is very modest processing temperatures. Because of its wide applica- important because it has orders of magnitude effects on the abil- tion and acceptance, this discussion is limited to the so-called ity of solder to creep under load. Lampe (1976) has found that near-eutectic (63-37 and 60-40) tin-lead solder that has a melting these solder aging processes approach equilibrium in a matter of point of approximately 183oC. a few days at 100oC, or a few months at room temperature. Solder Metallurgical Considerations. Before exploring the Solder Fatigue Strength. For most electronic packaging ap- mechanical properties of room-temperature solder, it is impor- plications it is not a single high stress event that breaks a compo- tant to note that molten solder is an extremely good solvent, and nent solder joint; rather it is repeated load applications that result readily dissolves metals such as copper and gold that it comes in in fatigue failure of the solder. contact with. Although this contributes to good surface wetting Figure 4 presents representative fatigue-life data for 63-37 properties, the strength and ductility of solder, as with most metals, SnPb solder taken from IBM test results (Wild, 1975). The plot are strongly affected by small concentrations (1 to 5%) of con- illustrates the typical dependency between cycles-to-failure and taminating elements (Berg and Hall, 1979). This makes solder- the level of total plastic strain introduced into the solder (total joint strength quite sensitive to the purity of the resulting solder peak-to-peak strain range per cycle). Note that the level of dam- in the joint, which is very dependent on the surface metals of the age (number of cycles to failure) is an extremely strong function joint and the length of time the metals were exposed to molten of the cyclic strain range. Doubling the strain range can reduce solder during the soldering process. For the purposes of this the fatigue life by nearly an order of magnitude. This linear log- paper it is assumed that good soldering practices are employed strain-versus-log-cycles-to-failure dependency is characteristic of to minimize the contaminant levelsthis includes removing gold ductile metals (Mon and Ross, 1982, and Aldrich and Avery, plating in a pre-soldering tinning operation, and employing fast 1970), and is quite independent of temperature. Figure 5 pre- soldering operations with high purity solder. It is also assumed sents data from Solomon (1986) illustrating the temperature in- that good surface wetting is achieved. dependence of the solder fatigue properties measured under vari- Upon solidification from the melt, solder is composed of a ous isothermal conditions. matrix of lead-rich and tin-rich grains that give solder its granu- In most electronic packages, the principal strain in solder joints 122 / Vol. 113, JUNE 1991 Transactions of the ASME Fig. 5 Fatigue endurance of 60-40 solder versus solder temperature Fig. 7 Strain rate versus stress for Sn-Pb 63-37 solder at room tem- (from Solomon, 1986) perature stant of the strain relief elements, and the chemical composition and size of the solder grains. Figure 7 illustrates the strong sensitivity of solder strain rate (creep rate) to the solder stress resulting from the load
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