We have had a chance to look at low-temperature solder alloys and indium in particular. Now it is time to look at high-temperature solder and braze alloy options. Bernie Leavitt is our Senior Product Specialist for high-temperature solder and braze alloys. Bernie has some important observations about reliability in high-temp alloys used for hermetic sealing and bonding.
"When we think high reliability we think of products that need to have a very low risk of failure. For our gold and braze products used in hermetic sealing and bonding, it means much more than that. It also needs to stand the day-to-day operations which can include extreme conditions in harsh environments.
Products made by aerospace companies, for example, require high reliability.
Thermal cycling is experienced in space satellites in temperatures that range from -170C to 123C because their orbit takes them from being in direct sunlight to being on the backside of the earth. Solder or braze needs to withstand the coefficient of thermal expansion (CTE) of the base materials. High silver-bearing braze alloys perform very well in these applications and have a tendency to outlast the base materials they are bonded to.
Creep resistance is a positive attribute of gold alloys because they are resilient enough to hold their form through thermal cycling and mechanical stresses, where other alloys will deform to relieve stresses. 80Au20Sn performs are ideal for high power LED die-attach applications where the solder is exposed to quick heat up and designed to dissipate heat quickly (which adds stresses when turned off).
Joint strength is critical and there are a couple of factors that contribute to this:
• For high reliability, an alloy needs to wet well to limit voids. Optimizing all potential bonding surfaces increases shear strength and decreases the chance of pathways for leaks.
• A strong intermetallic layer is where the solder/braze material and base material form a new alloy. Typically, with nickel coated base materials you only want a 1-2µm intermetallic layer. The intermetallic thickness is the result of the amount of time over liquidus during reflow, so having a good profile is key.
Here’s a list of some important high-reliability solder and braze alloys that the Indium Corporation offers:
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Indalloy®182 80Au20Sn
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Indalloy® 183 88Au12Ge
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Indalloy®193 72Ag28Cu
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Indalloy®207 99.99Ag
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Indalloy®200 99.99Au
If you have an application where you need a recommendation on the right alloy or if you are looking for help with your reflow process, the Indium Corporation Tech Support Team has engineers that have hands-on experience to help optimize reflow profiles and troubleshoot issues. Please contact me, Bernard Leavitt (bleavitt@indium.com), if you have additional questions or like to get some literature on these alloys."