MCEN5228-011/4228-011: Microsystems Integration
Homework #3, Due Date: Monday, February 13
The Word file of the homework is posted on the class web site. Email your PDF or Word file with answers to before the class on the 13th. There is a penalty, e.g. 5 to 25 points, for late submission.
Name: ______, Email: ______
1. A calculation example shown below was discussed in the class. You calculated two cases in the last homework. Estimate fatigue lives in the additional two cases specified below.
o The solder joint height is reduced from 100 to 10 µm. What is the desirable chip size if we want the solder fatigue life to reaching about 10,000 cycles. (5 points)
o For the case with Si-on-FR-4 PCB, we can bond the silicon chip and the PCB together by using underfill epoxy. The corresponding solder fatigue life would be increased substantially since PCB’s effective CTE is reduced. Estimate the reduced effective CTE if the fatigue life is about 10,000 cycles for the Si-on-FR-4 bonded by the underfill epoxy. Note: Silicon chip is very rigid, so its CTE would not be affected by the underfill epoxy. (10 points)
2. Estimate the power dissipation level allowed for a chip flip-chip assembled on an alumina substrate cooled by forced air with h = 20 W/m2 oC. The convective cooling on all other sides is negligible. The maximum junction temperature allowed is 100 oC. Use class notes for the Case 2: Flip-Chip vs. Wire Bonded for other materials and dimensions. The alumina substrate is 2cm X 2cm X 0.75cm; its K=30 W/m oC. The ambient temperature is 25 oC. Is the spreading thermal resistance from the solder joints to the alumina substrate negligible? In addition, estimate the fatigue life of the solder joints. (15 points)
3. Graphite heat spreader made by GrafTech is being widely used for smartphones and tablets. As shown below, graphite’s in-plane thermal conductivities can be very high; however, its through-plane thermal conductivities are very low. A) Please identify one advantage resulting from the low through-plane conductivities for a smartphone. B) Also shown in the table, the thickness range of high thermal conductivity graphite (k=1,500 W/mK) is from 0.017 to 0.025mm. It is too thin to be useful for many applications. Why do we want to have a thick graphite heat spreader? (10 points)
4. Different thermal management solutions shown below were discussed in the class. If needed, you can read the paper posted on the supplementary page in the class web site. Compare the results with (a) all thermal pads off and (c) inner and outer thermal pads on. Answer the following questions: A) why are junction temperatures reduced with the pads on? B) Why are skin temperatures on the backplate increased with the pads on? C) Is backplate’s thermal conductivity important? (10 points)