Three of the most commonly-used, and effective, brazes used in the power and RF hermetic packaging process are:
- 72Ag28Cu
- pure silver
- OFHC Cu
There are two key reasons cited for these brazes’ widespread usage; they are user friendly and they provide finished goods reliability. Let’s explore more deeply.
- 72Ag28Cu (melting point: 780°C): This is the most commonly-used alloy for hermetic sealing of high-reliability packages. It wets and reflows well in a forming gas atmosphere, making it ideal for glass-to-metal seals that rely on capillary action for good seals. Capillary Blocks® or cut wire can be used instead of a frame preform to save some cost when brazing the base plate to the frame. Because it wets so well, this alloy can cause problems like flashing or wicking, where braze gets pulled away from joints if the brazing process includes excessive time over liquidus during reflow.
- 99.99Ag (melting point: 962°C): Pure silver braze is typically used to assemble packages that will see multiple reflows during the brazing process. Sometimes companies will braze the glass seal to leads, then reflow the ceramic feed-through to the frame or base on the next reflow typically using 72Ag28Cu.
- 99.99 OFHC copper (melting point: 1,083°C): Pure copper braze is the most economical option; and it wets and reflows well. The heating limit of most belt furnaces is approximately 1,100-1,200°C, near the recommended brazing temperature of pure copper. At this extreme, maintaining temperature accuracy and consistency is challenging as heat is lost during the process, while production material is run through the furnace. For this reason, product spacing and graphite boat mass are critical factors. Another hurdle is that most packages are made from Kovar, so warping can be an issue at these high temperatures.
Summary:
These three brazes are popular because they’re easy to use and they get the job done. There are some possible issues with each, but we know how to address them. Performing test runs on new packages and understanding design limitations can help you make the right decision on which brazes to use in each situation.
If you have any questions or are looking to get some help on a new or existing project, please email me or give me a call. I can even pay you a visit, if needed, to help with an evaluation.
Email: bleavitt@indium.com
Phone: 315-534-8123
My next post will cover some key issues regarding brazing reflow profiles.