An Overview of Fundamentals
BY LOH KIM YEN
Power microelectronics devices generally are used in automotive packages in which the current-flow density is high. If not dissipated properly, the heat generated by high current densities can damage the package. A substrate or lead frame with an exposed heat sink is used for heat dissipation and as the die-attach pad, and soft solder generally is used as the joint because of its high thermal and electrical conductivity.
Solder Wire Materials
Hard solder is gold-based eutectic solder such as Au/Sn20, Au/Si3 and Au/Ge12. It features higher melting temperature and mechanical (tensile) strength than the others and is free from thermal fatigue failure when subjected to temperature cycling.
Soft solder is lead-based solder, e.g., Pb/Sn1/ Ag1.5, Pb/Sn2/Ag2.5 and Pb/Sn5. Soft solder exhibits lower mechanical (tensile) strength compared to hard and intermediate (tin-based) solder.
Intermediate solder is tin-based, e.g., Sn/Sb8 and Sn/Ag25/Sb10 (J-Alloy). The material shows a behavior between that of hard and soft solder and has better fatigue resistance (fracture strength) than the latter.
Lead-free solder also is getting more attention. The materials chosen to replace lead must be non-toxic; be available in sufficient quantity for global needs; have similar or better properties to lead-based solders regarding ductility, wettability and melting temperature; and be workable on existing soft-solder die-bonder equipment.
Lead-based Solders
Tin/lead-based (Sn/Pb) solders are most commonly used in the semiconductor industry. With Sn/Pb, Ag can be added as another constituent material to improve the fracture strength of the solder during aging (temperature cycling). Pb-based solder also can be doped with impurities to improve viscosity and wetting or oxidation. Solder wetting can be determined from the wetting angle of the solder.
During the solder dispense process, solder will react with the substrate to form intermetallic layers. In the case of a Cu substrate, the intermetallic layers formed are Cu3/Sn, Cu6/Sn5 and Ag3/Sn. Sn is the only reactive component in the intermetallic reaction, and the reaction stops when most of the Sn has combined with substrate elements to form intermetallic layers. With Pb-based solder, the microstructures of Cu3/Sn, Cu6/Sn5 and Ag3/Sn are relatively fine and homogeneous. The grain size is small, resulting in better resistance to solder cracks and voids.
For power microelectronics, the substrate or lead frame is different from the typical lead frame for epoxy die-attach. The substrate has a heat sink, normally welded on, that also serves as the die pad. Examples of substrate and heat sink material are Cu/Fe2/P and Cu/Fe0.1/P, respectively.
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Although solder wetting behavior with Ag is good, the dissolution rate of Ag in solder material is high. Owing to the thin Ag plating (4 to 12 µm), there is a tendency for the entire Ag plating to be “etched off” by the solder material.
Contamination in the solder wire refers to metallic alloying additions. The impurities, which cannot be eliminated totally from the solder wire, can stem from the ores used as raw material. Contaminants like aluminum and zinc are very detrimental to the performance of the solder while impurities such as sulfur can cause certain dewetting conditions.
 Figure 2. The dissolution rates of Cu, Ni and Ag in solder material. There is a tendency for the entire Ag plating to be “etched off.” |
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Oxygen will react with solder to form oxides such as lead oxide (Pb/O) or tin oxide (Sn/O2). Pb/O is red or yellow and its melting point is 888°C. Sn/O2 is white or gray with a melting point of 1,630°C. Oxygen combines mostly with Sn to form Sn/O2, which is stable oxide and cannot be removed even in forming gas (N2H2). It also is difficult to prevent solder oxidation during the die-attach process because the forming gas always has some oxygen impurities. Solder oxidation before die attach will increase the solder void percentage.
With Cu substrates, the copper will react with oxygen impurities in the forming gas to create copper oxide (Cu2/O):
4Cu + O2 Æ 2Cu2O (oxygen impurities)
Cu2/O + H2 = 2Cu + H2/O (forming gas)
For Ni-plated substrates, the nickel will react with oxygen impurities in the forming gas to create nickel oxide (Ni/O). Ni/O is green, stable and the rate of reduction (by forming gas) to form Ni at temperatures ~400°C is negligible. Therefore, it is essential to have Ni/P plating on top of the Ni layer to prevent the latter's oxidation.
For Ag-plated substrates, the silver will react with oxygen impurities in the forming gas to create silver oxide (Ag2/O), which is gray and can be reduced in forming gas:
4Ag + O2 = 2Ag2/O (oxygen impurities)
Ag2O + H2 = 2Ag + H2O (forming gas)
The soldering process requires clean surfaces so that the base metal may be exposed to the molten solder. During the soldering process, flux provides tarnish-free surfaces and keeps them in a clean state, prevents further oxidation of the base metal, and decreases the surface tension of the solder.
Flux must be removed after the soldering process. For a flux-free soldering process, a vacuum or inert gas environment is needed to prevent base metal oxidation.
Forming gas always contains a certain amount of oxygen impurities that, together with the outside air, will tend to cause substrate oxidation. With the presence of forming gas, however, reduction of substrate oxides will occur.
During the solder joint formation, present materials and the temperature control this liquid solder/substrate/die interaction. Solder alloy composition, substrate surface plating and die backside metallization significantly influence the soft solder die-attach process.
 Figure 3. Die-attach structure on a substrate. Soft solder will form intermetallic phases with the substrate and die metallizations. |
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In the soft solder die-attach process, a higher temperature improves the solder spread and backflow. However, at higher temperatures, solder bleed-out is difficult to control because solder viscosity and surface tension are reduced and possibly lead to solder overflow.
Solder joint reliability is characterized by the degree of void formation and the fracture behavior of the joint after aging. An important mechanism that determines the fracture behavior of the solder joint is thermomechanical fatigue, which can lead to open-circuit failure. Thermal fatigue occurs as a result of the fixed strain that is generated in each cycle due to the coefficient of thermal expansion (CTE) mismatch of the materials present. The fatigue life of a solder joint is given by the Coffin-Manson relation.
Solder composition, morphology and reaction with the substrate will affect the fatigue life of the solder joint. Voids cause localized heating and stress concentration. The thermal resistance (RT) of the solder increases as the package is thermally cycled.
Thermomechanical stress, generated during temperature cycling, will reduce solder joint fatigue life. It can be determined that as solder thickness increases, thermomechanical stress decreases until reaching a point where further increases in solder thickness will have no significant affect on such stress. Thermal fatigue cracking generally initiates at points of high strain. This region eventually will accumulate high tensile strains perpendicular to the shear direction. When a critical amount of strain is reached, separation within the persistent slip band will occur.
Conclusion
The solder composition, the substrate finish, thermal issues, the use of flux and control of oxidation are some of the many subtleties that define the ideal process for a given application.
Loh Kim Yen may be contacted at Infineon Technologies Asia Pacific Pte Ltd., 168 Kallang Way, Singapore 349253; 65 68400956; Fax: 65 68400432; E-mail: [email protected].
REFERENCES
1. J.H. Lau, Solder Joint Reliability: Theory and Applications, Van Nostrand Reinhold, 1991.
2. H.H. Manko, Solders and Soldering: Materials, Design, Production and Analysis for Reliable Bonding, McGraw-Hill, 1979.