IC manufacturers typically specify the maximum allowable force/pressure on the package. The spring attachment load needs to be under this maximum allowable limit.
The thermal impedance of a PCM(Phase Change Material) or Gap Filler pad depends on the pressure applied. More pressure equals lower thermal impedance and therefore better thermal performance. If the applied pressure falls below the published minimum range for a TIM, the performance can be poor. In a case like this, the use of grease, which is far less sensitive to pressure, should be considered.
Typically the spring rate for a compression spring is consistent when compressed 30% to 70% of the distance from free length to solid height. We recommend designing to this compression range. The spring rate would be lower for compression to less than 30%, and higher for compression greater than 70%. Also, under high compression, the spring may take a set and the load would actually be reduced.
© 2009 Alpha Novatech, Inc.