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Solder Selection for Photonic Packaging
December 31, 1969 |Estimated reading time: 2 minutes
By Brian O'Neill
As the electronics and photonics worlds continue merging, contract electronics manufacturers (CEM) involved in traditional SMT assembly will be called upon to integrate optoelectronic component assembly into their operations. While there are many challenges posed by the assembly of such complex hybrid packages, some of the biggest revolve around how to assemble a diverse array of ceramic, glass and metal components within a package. Three challenges are discussed:
Soldering to Au Films. Gold coatings are used extensively on photonic packages because of the reliability requirements demanded by the fiber optic industry. While this provides optimal conditions for fluxless assembly, the dissolution of gold into tin-based solders can cause solder joint embrittlement. At normal reflow temperatures, Sn60 can dissolve gold films at a rate of 1 æm per second. Once the solder joint's Au content exceeds ~5 wt. percent, Au/Sn4 crystals form within the solder, effectively embrittling the joint and risking premature failure especially in the cases of extreme temperature cycling.1
Indium-based solders (In/Pb, In/Ag, In/Pb/Ag) are good alternatives to tin-based solders when soldering to Au films. It has been demonstrated that a 6 æm thick gold film can withstand molten 50In/50Pb at 250°C for 15 minutes.2 While gold will dissolve into In/Pb systems, a layer of Au/In4 forms at the joint interface, acting as a barrier to further Au film dissolution. Further, the Au/In4 intermetallic that will form remains ductile and allows for an effective solder joint with good fatigue life.
Fiber Soldering. One of the more difficult joints to make is the alignment and soldering of an optical fiber within a package. With tolerances in the submicron range, extremely low creep solders must be used. With a tensile strength of ~40 Kpsi and a modulus of ~59 Gpa, eutectic Au/Sn is the best choice for this application. With the aid of a pre-solder chemical etch or a reducing atmosphere for reflow, this alloy can provide a fluxless solder process that yields creep-resistant solder joints.
CTE Mismatch. Photonic components require the joining of a diverse range of components - ceramic submounts, optoelectronic semiconductors, optical fibers, detectors, thermistors - all within a kovar package. Once hermetically sealed, the device must withstand temperature cycling of -40° to 85°C, resulting in coefficient of thermal expansion (CTE)-induced stress on all component interconnections. However, with their high ductility, indium alloy solders can accommodate these CTE mismatches while providing excellent fatigue life and good thermal conductivity.
In a study of silicon flip chip interconnections to alumina substrates that compared Sn/Pb solder to indium alloys, the assemblies were subjected to thermal shock tests (liquid nitrogen to room temperature) with electrical resistance measurements used as an indication of defects.3 After 88 cycles, the resistance of the eutectic Sn/Pb joints began to increase sharply. Resistivity of In/Sn and In/Pb joints was stable until ~180 cycles while the resistivity of pure indium interconnections remained unchanged through the life of the test (300 cycles).
Use of the appropriate solder alloy for a given solder step permits the process engineer to manage diverse materials issues while minimizing yield loss because of defective solder joints.
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References
- Jacobson, D.M., Humpston, G., "Gold Coatings for Fluxless Soldering," Gold Bull.,1989, 22(1).
- Yost, F.G., "Soldering to Gold Films," Gold Bull., 1977, 10(2).
- Shimizu, K., et al., "Solder Joint Reliability of Indium Alloy Interconnection," JOEM, 1995, 24(1,).
Brian O'Neill may be reached at AIM,25 Kenney Dr., Cranston, RI 02920; (401) 463-5605, ext. 5215; Fax: (401) 463-0203; E-mail: boneill@aimsolder.com; Web site: www.aimsolder.com.