Typical applications for the family of SST Vacuum Reflow Systems are: Package Assembly, Lid Attach, Die Attach, Glass to Metal Seals, and Brazing. These process profiles share a common sequence.
Most process cycles begin by removing as much of the ambient atmosphere as possible and diluting what remains with an inert gas. Vacuum is turned on until the chamber reaches a low pressure. Inert gas, usually dry nitrogen, is then introduced into the chamber. This vacuum/pressure step can be repeated to further increase the cleanliness of the sealing environment and reduce the overall moisture level.
A vacuum pre-bake provides the following benefits:
- It removes contaminants present in the process chamber’s ambient atmosphere.
- It removes contaminants adhering to the surfaces of the piece-parts.
- It removes contaminants trapped interstitially in the piece-parts.
- It improve the thermal gradient in preparation for the final seal.
The pre-bake temperature is typically set to 50C below the reflow temperature of the bonding material.
The Final Seal dwell is the interval when the temperature is maintained at its highest level. The time and temperature parameters are intimately related. By raising the temperature, the dwell time can be shortened. By lengthening the dwell time, the temperature can be lowered.
The relationship of temperature and time becomes important when heat-sensitive components are involved. For most applications, a brief dwell at high temperature is preferred over a long dwell at low temperature.
For most solders and braze materials, the optimum process temperature is about 50C above the flow point. For most glasses, the optimum process temperature is about 80C above the softening point. For most solders and braze materials, the optimum dwell is about 2 minutes. For most glasses, the optimum dwell is about 1-1/2 minutes.
The Cool Down is the final phase of the process cycle. For most solders and braze materials, the components are cooled un-assisted until 10C below the flow point. For most glasses, the components are cooled un-assisted until 20C below the softening point.
Assisted cooling is performed by continuously flowing inert gas into the chamber, with the exhaust valve open. It is recommended to open the process chamber when the temperature is below 200C to prevent graphite from reacting with oxygen in the air.
Typical process profiles share a common sequence: Chamber Preparation, Vacuum Pre-Bake, Final Seal and Cool Down. These steps are adjusted in various ways to achieve desired results for the various processing applications.
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Sr. Electrical Engineer
SST Vacuum Reflow Systems