Creating an airtight or hermetic seal is a critical-- and an especially challenging-- requirement for high-reliability packages. The reflow profile can make or break the manufacturing process.
Many times a reflow profile is inherited over the years and as long as it works, it is not adjusted. However, for braze alloys there may be some changes you can make to optimize your process and increase your yields.
An important part of the braze furnace profile is the dwell cycle, which is the amount of time that parts plateau just under the liquidus temperature. Typically the target temperature for the dwell cycle is 30-50c below liquidus temperature. The purpose of the dwell cycle is to have all the parts at the same temperature to limit the amount time the parts will be exposed to the liquidus temperature. If you have certain production runs with a couple of the preforms not re-flowing, you might need to increase the dwell cycle of the profile. Depending on the furnace and number of zones, this can be accomplished by:
- Adjusting the profile
- Adjusting the belt speed
- Adjusting the distance of the parts on the belt
We recommend that the reflow portion of the profile last around one minute. This gives braze enough time to capillary into the joints and bond with the nickel metallization. If you have a slightly longer liquidus portion of the profile-- up to 2 minutes-- and you have strong joints with high yields, that is fine. Extended time over liquidus can allow braze to get through the nickel plating, causing delamination on ceramic feedthroughs, which leads to leaks and weak braze joints, because braze doesn’t wet to the ceramic.
Other symptoms of excessive time over liquidus includes wicking or flashing of the braze material. This is where the braze capillaries into the joint then starts wetting outside the joint on the nickel plating. This decreases the braze volume in the joint, making it more prone to leaks and weak joints.
Conclusion:
The 72Ag28Cu profile provided can give you a good reference point for evaluating your current manufacturing profiles. We understand that there are a number of variables that impact production yields along with profile thermal mass, belt speed, and graphite assembly’s that hold the hermetic packages in place in the furnace. If you would like to learn more, contact me and I will be glad to help. bleavitt@indium.com