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48

Chip Scale Review May • June • 2019

[ChipScaleReview.com]

After the EMC pellets are loaded, they

are melted and injected into the cavity with a

pressure limit of 4MPa. During this operation,

the sample is clamped at 950kN, which

increases to 1100kN if the mold opening

exceeds a threshold of 10µm, or when the

compaction phase is activated. This high pre-

clamping force, or relatively low increase in

clamp force of 16%, is possible due to the

simple layout of the demonstrator sample.

As explained earlier, the die population and

interconnect density determines the maximum

amount of clamping that can be applied.

Although this simple demonstrator product

has a die population of only 35%, the pressure

that can be applied on the die is high as the

die is directly supported by the bond layer and

carrier (P3 and K3 in

Figure 5

). Therefore, a

high pre-clamping force can be used, which is

beneficial for the process.

The final result of the molded demonstrator

is shown in

Figure 9

. The injection point is

located at the lower right side, and the EMC

flows from the lower right to the upper left.

The notch for rotational alignment is on the

left side. Further, one can see the clamp edge

of themold to the glass carrier, (P1 in

Figure 5

)

and an overview of the exposed dies. Clearly,

the die surfaces are kept exposed. All dies are

visible, but more important, however, is that no

discoloration is visible, and therefore, no signs

of flash. This indicates a good uniformity of

how the mold tool clamps onto the die, and

thereby the achievement of a stable clamping

process, for which the dynamic clamping

was setup. Secondly, it is visible in the lower

left picture that the mold cap and die are on

the same height level, which is beneficial for

further processing, but also shows that the

loading on the die was not too high, as the die

would then sink into the release foil leaving

an imprint in the mold cap. So this operation

again shows the purpose of applying dynamic

clamping. Finally, it is important to notice that,

although clamping of the mold tool occurred

directly onto the glass carrier, this surface is

clean and shows no damage.

Summary

• This paper explains that both EMC

properties and clamp force control

are dominant topics in the field of 12”

wafer-level transfer molding.

• A basic method to estimate the effect

of EMC choice on the resulting

wafer-level warpage after molding is

demonstrated.

• The impact of clamping dynamics

during exposed die transfer molding

were discussed and accompanying

control strategies were introduced.

• Finally, the process of exposed die

wafer-level packaging is demonstrated

on a 12” exposed D2W sample.

• With both warpage control and the

exposed dies capabilities, transfer

mol d i ng a t t he wa f e r l e ve l i s

shown to be a serious alternative to

compression molding

Acknowledgments

We wou ld l i ke t o t ha n k Hit a ch i

Chemical Co. Ltd. for delivering epoxy

molding compounds and release film for

this research.

References

1. K. Kwon, J. Y. Chung, D. Lee, S.

K. Kim, “Wafer warpage control by

epoxy molding EMCs for wafer level

package,” IEEE 67th Elec. Comp.

and Tech. Conf. (ECTC) (2017).

2. S. P. Timoshenko, Jour. Opt. Soc.

Am. 11, 233 (1925).

3. S. H. M. Kersjes, J. L. J. Zijl, W.

G. J. Gal, H. A. M. Fierkens, H.

Wensink, “Exposed die wafer-level

encapsulation by transfer molding,”

Elec. System-Integration Tech. Conf.

(ESTC), (2016).

4. W. Gal, et al., “Wafer-level packaging

(WLP) by transfer molding,” ISIT

Conf., Belgium, 2009.

Table 2:

Molding parameters used for the

demonstrator sample.

Figure 9:

A molded D2W demonstrator sample. Us-

ing wafer-level transfer molding, an exposed die 12”

FOWLP on a glass carrier was achieved.

Biographies

Sebastiaan H.M. Kersjes is the Manager of the process technology group at Besi Netherlands B.V., The

Netherlands. He has over 12 years of experience with back-end processes for the semiconductor industry, focusing

on packaging processes such as molding, trim and form, and singulation. Next to managing the process technology

group, he maintains the international R&D network for the packaging division of Besi, and has cooperated in

starting up several relevant European projects. Mr. Kersjes holds a MSc degree in Mechanical Engineering from the

Technical U. of Eindhoven; email

Sebastiaan.kersjes@besi.com

Jurrian L.J. Zijl is a Process Scientist at BESI Netherlands B.V. He has 18 years of research experience in the back-end semiconductor

industry. He contributed to many new developments on molding, trim and form, and laser singulation equipment. He holds a MSc

degree (’97) in Mechanical Engineering and a PhD (’01) in fluid dynamics from Delft U. of Technology in the Netherlands.

Niels de Jong is Product Manager Molding at Besi Singapore, Taiwan branch. After a brief start as a physics teacher, he made the

jump to production machine building and has been doing that ever since. He has been working for Besi for 8 years now, mostly in the

Asian region, where he is currently also stationed.

Henk Wensink has been a Senior Manager at Besi Netherlands B.V., with 14 years of experience in the semiconductor packaging

industry. He currently holds the position of team leader at the Wageningen U. and Research the Netherlands. He has MSc and PhD

degrees in Applied Physics.