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Processing Dow Corning® Brand Silicone Encapsulants — Vacuum Dispense
Vacuum dispense provides the opportunity for high-speed, high-volume production but it also presents several material issues not seen at ambient conditions.
Micro-BGA Vacuum Dispense Pattern for TV-46
The material has to be stable under vacuum and it has to flow well enough to complete the underfill during the time it takes to break vacuum.
The standard machine is set up with three chambers. Both Camalot and Asymtek offer vacuum dispense options. The first is the input chamber, which cycles the strip in from the feed magazine. The second or main chamber is where the dispense takes place, and the third chamber is where the vacuum is released, forcing the encapsulant underneath the die and around the standoff, whether it is a pad or nubbins.
With a larger size die, it becomes very difficult to underfill completely in one pass. A modification of the atmospheric dispense is recommended. This uses a dispense pattern that goes around three sides of the die, but stops short of the leading edge, allowing the material to flow under and along the three sides of the die without meeting on the fourth side and trapping a void.
Process - 3-side dispense (in black), wait (variable amount of time), then dispense fourth side dispense (in red)
long way=streets, short way=alleys
Key points:
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It is important that all edges of the die are wet before breaking vacuum to prevent air entrapment.
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The pattern should be adjusted, depending on the shape and size of the die in use.
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Lamination of coverlay and storage condition of the strip can play a large part in a successful encapsulation.
3-Sided Dispense
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Effective on larger parts, underfills by capillary action
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The use of vacuum eliminates air and the formation of voids after cure
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he fourth side may have to be dispensed depending on the quality of the fillet formed on the fourth side of the die
The dispense pattern is very critical. If material is dispensed to the leading edge of the die, there is danger the material will flow around and meet itself, resulting in a void (example 1).
If dispense is stopped a certain distance (d) before the leading edge (example 2), the material will flow without trapping a void and completely underfill the part.
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