Design Rule Study of Compression Molding Process on Polymer Cavity Packages
Authors: Honda Nao , Hakone Yoshihiro 
Published: January 2025

INTRODUCTION

It is commonly cited that packaging accounts for 80% of the cost of a MEMS device, and a lot of wafer level package (WLP) processes are being developed to address the issue. Amongst those, polymer cavity package has been identified as a way to reduce package cost. In these applications, epoxy dry film resist (DFR) is one of the most promising materials to fabricate cavity structures. The simple procedure, consist of laminating a wall DFR structure, followed by a photolithograph step. This is followed by a second laminating step, in which a DFR cap layer is attached to the wall layer, and then completed with a photolithograph step. In addition to the cost advantage, another advantage is low package height. Silicon or glass wafer caps are typically in the 200-400 micron thickness range. DFR, on the other hand, is usually 15-50 micron thickness. Thus, thinner DFR thickness contributes to a lower profile cavity package. For this reason, DFR polymer cavity is getting more major on the package method for MEMS including SAW and BAW filters. However, due to the thinner cap to respond to further lower profile trend, we have a limit to the cavity width that can withstand molding pressure. Therefore, low pressure molding process, and also suitable DFR and EMC(Epoxy molding compound) are required.

In this paper, EMC filling test and polymer cavity molding resistance test were conducted in order to verify the low pressure compression molding concept for polymer cavity package.

IMPLEMENTATION

For a long time, the transfer molding process, in which EMC is melted by heat and extruded into a mold cavity with a plunger, has been widely used as a semiconductor molding process. In recent years, on the other hand, compression molding process, in which chips or a wafer is immersed into heat-melted EMC to reduce damage to the copper wires, has begun to spread. On transfer molding, a part of EMC must stick to the plunger, then it causes pressure loss due to the friction around the plunger. Actual molding pressure of transfer molding at low pressure region is no longer equal to the pressure setting after several dozen shots. For this reason, it is said that lower limitation of the transfer molding pressure is around 4MPa to give stable molding quality. On the other hand, compression molding can give very stable pressure profile even under the low pressure region like 1MPa since it doesn’t give friction or pressure loss. Therefore this experiment was implemented with compression molding.

EMC Filling Test

Standard EMC Version A and high fluidity EMC Version B (both are produced by Sumitomo Bakelite) were used for confirming the filling performance on low pressure compression molding. After flip chip bonding of a general flip chip test element group, compression molding was performed under the following conditions. Die size/thickness: 5 x 5 mm/100 micron, Bump height/pitch/diameter: 50 micron /200 micro /90 micron, EMC thickness: 500 micron, compression molding condition: 1, 2 and 3MPa, 175degC for both Version A and B. After molding, the substrate was polished and removed, and the presence of filling defect around bumps was evaluated. Version A didn’t generate filling defect at 3 MPa, however, a micro void (filling defect) was observed at 1 and 2MPa. On the other hand, Version B didn’t generate filling defect even at 1 MPa.

Polymer Cavity Molding Resistance Test

We used SU-8 3000CF DFR (Produced by Nippon Kayaku / MicroChem) as a permanent epoxy DFR for both wall and cap layer and DXL Series (Produced by Teikoku Taping System) as a full automatic laminator. We fabricated various cavity width cavities which consist of wall and cap layer on a wafer. The wall height was 20 micron, wall widths were 10 micron and cavity widths were every 10 micron between 50 micron and 500 micron. On the other hand, we used 20 micron, 30 micron and 45 micron thick cap layer.

After compression molding at 1, 2 and 3MPa, we obtained x-section SEM images and firstly measured maximum cap displacement (Î´max) which is cap sagging due to molding pressure. The compression molding condition was consistent except for molding pressure; 1, 2 and 3 MPa(pressure), 175C(temperature), for 172sec(compression time) and 300sec(cure time). Then secondly we found maximum cavity width which gave less than 5 micron max . For example, here is a 1 MPa and 20 micron cap case, Î´max were 2.7 micron at 240 micron width cavity and 5.3 micron at 250 micron width cavity, so we decided maximum cavity width under that condition was 240 micron.

RESULTS AND DISCUSSION

We successfully fabricated polymer cavity package with 460 micron cavity width, 45 micron thick cap and 200 micron thick EMC, then it was confirmed that the package satisfied with MSL (Moisture Sensitivity Levels) 1.

On EMC filling test, standard EMC Version A could not be used for 1 or 2MPa compression molding since it generated micro voids at 1 and 2MPa. However, high flowable EMC Version B solved the issue and successfully showed high filling performance even at 1MPa.

On molding resistance test, 3MPa with 20 micron only achieved 150 micron cavity width, however, 1MPa with 45 micron achieved 460 micron. All data were plotted. The result was strongly supported by the theory of both ends fixed uniformly distributed load as shown below.

Î´max = WL3/384EI, where Î´max is displacement at the center(mm), W is total load(N), L is length of the material(mm), E is young modulus of the material(N/mm2) and I is moment of inertia(mm4).

As you can see in the formula, cap layer displacement (Î´max) is proportional to cube of the ratio of material length (L). So it could be said that cavity width has the greatest impact to molding resistance. Also, reducing molding pressure and increasing young modulus of the material, which can correspond W and E respectively, are promising when improving mold resistance. Furthermore, low temperature molding is also suggested to improve performance since young modulus of the DFR is higher at low temperature.

CONCLUSIONS

We successfully fabricated extremely wide polymer cavities up to 460 micron width by using 45 micron SU-8 3000CF DFR for cap layer and following low pressure compression molding. 1MPa low pressure compression molding with conventional EMC may generate micro voids on the filling test, however, this insufficient filling performance at low pressure was able to be solved by using high flowable EMC. In this paper, low pressure compression molding demonstrated that molding resistance and molding filling ability are compatible. Significant advantages of compression molding were not only that very stable pressure profile even under the low pressure, but also that EMC thickness can be easily controlled thinner than transfer molding. Considering these facts, low pressure compression molding has proven to be a suitable process for polymer cavity packages.