Finishing up our look at the 2013 IMAPS Device Packaging Conference
Leti examined the reliability of die to wafer bonding using copper/tin interconnects. Above 232 C tin rapidly reacts with copper to produce higher melting point intermetallic compounds.
They studied thermal cycling, TC (500x -40 to +125 C) and high temp storage, HTS (84 hrs at +125 C) for both underfilled and non underfilled Cu/Sn joints and found that TC has a more pronounced negative effect on yield and electrical performance than HTS.
After thermal cycling one finds increased growth of the Cu3Sn layer and cracks at the Cu/Cu3Sn interface. Underfilling has a positive impact on yield and electrical performance.
Leti also addressed the thermal and mechanical challenges of 3DIC integration.
In terms of mechanical issues, everyone agrees that BOW is the problem. Potential solutions include:
- compensation layers on both sides to match stress.
- compliant interconnect
- increase interposer thickness
- control polymer encapsulation.
The following is an interesting plot that they use to make the point that a thicker substrate is better for mechanical stiffness. IFTLE thinks this is a bit of an over simplification since it depends on the modulus of the insulator layers and the encapsulation and their thicknesses. Certainly the general conclusion is correct.
Dow Chemical and Fraunhoffer IZM presented new data on BCB based temporary bonding adhesive. The process flow for temp bonding is shown below:
Wafers can be bonded at 80 C with a bond time of ca. 1 min. and subsequent oven curing at 210 C for 1 hour. They find no alignment shift after curing. Wafers show no voiding or delamination after 1 hr at 325 C indicating excellent stability for all backside processing.
Mechanical debonding at room temp needs no irradiation or chemical treatments to release the thinned wafer. The mechanical debond is reported to be “residue free.”
If bumped first, one can coat twice to build up greater than 80um of BCB. Mechanical peel off reportedly does not delaminate any bumps. Subsequent cleaning is done with IPA.
Dow has also developed a pre-applied underfill materials set. We were shown 25um thick film roll that was 330mm wide.
Dow also gave an update on the new BCB XP 120201 a new low stress version of their positive tone, aqueous developable 6500 series. It has reported cure temp less than 180 C and an elongation at break now greater than 25%.
SCP, one of the acknowledged leaders in fan out WLP announced the use of a new organic dielectric which results in more robust mechanical performance and now allows the packages to pass -55 to +125 thermal cycling. They have gotten 11 x 11mm 28nm die in 14x14mm substrates on 0.4mm pitch to pass reliability.
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