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Chip Scale Review March • April • 2019

[ChipScaleReview.com]

Failure relief in WLP and PLP polymer layers

By Robert L. Hubbard

[Lambda Technologies, Inc.]

o avoid material stresses in

fan-out wafer-level and panel-

level packages that create

warpage, cracking, delamination, and

thermal instabilities, the use of coefficient

of thermal expansion (CTE) matching and

the lowest possible process temperatures

has been a focus with this technology. Data

is provided to show that this approach is

actually causing more stress and crack

propagation than is necessary, especially for

large wafers and panels. An understanding

of the source of these chemical, mechanical,

and thermal instabilities is offered with

methods for avoiding these issues. Reduced

warpage and bow, increased fracture

energies, and wide-area film uniformity

data will be shown to be possible on the

largest of substrates with the use of low-

temperature curing by variable frequency

microwave curing. A method is also

described for increasing the speed and

reducing the cure temperature even further

with customized resins.

Introduction

T h e r e a r e a s m a n y d i f f e r e n t

constr uctions and processes used in

wafer-level and panel-level packaging

(WLP/PLP) as there are practitioners,

but a common st ack-up consists of

an ar ray of silicon die encapsulated

in a thermoset polymer with multiple

redistribution layers of metal traces and

thermoplastic dielectric coatings. As

with most packaging designs, there are

silicon and metal layers with low CTE

values, and organic polymer layers with

high CTE values. This well-known “CTE

mismatch” creates stress at the interfaces

when temperatures are changed during

subsequent process steps and when the

products are in the field. Additional

stresses are developed in the polymer

layers during the chemical reactions

necessary in thermal polymerization steps

(“curing”). An example of the layers used

in fan-out wafer-level packaging (FOWLP)

is shown in

Figure 1

(not to scale).

Many users of the chemicals in thermal

polymer ization are unfamiliar with

the sources of these stresses and the

mechanisms that create delamination,

cracking, part movements, and instabilities

in these materials. The resultant polymer

layers often do not have the expected

properties listed on the technical data

sheets from the material suppliers.

In many cases there are intentional

short cuts taken in the cure processing

temperatures and times to produce more

cost-effective but “good enough” quality.

These shor t cuts appear to produce

workable results at each step but contribute

to incipient failures at later steps or failures

in product lifetimes. The failures are not

always traced back to the “good enough”

choices made during processing and result

in costly poor reliability at the customer.

As the size of packaging increases from

single-die to multi-die packages, and now

to wafers and panels, these issues become

not only more numerous, but size specific.

New methods for producing very large

(and more cost-effective) panels lead to

more possible sources of failure in process

and product. A better solution would be

to understand the inherent properties of

the materials and to take steps to produce

reliable chemical, mechanical, and thermal

stability at each manufacturing step.

Polymer cure fundamentals

The following sections discuss some

fundamentals of thermal polymerization

that may not be widely known. The

critical nature of cure temperature is

common to both thermoplastics (for

redistribution layers) and thermosets

(for encapsulation) despite very different

cure mechanisms.

Thermoplastic dielectric layers.

Most polyme r d ielect r ic f i lms a re

thermoplastic, which simply means

that the supplied resin is an already

polymerized long chain structure that

has features along the chain that can

be changed in a manner that creates a

more rigid insoluble film. When cured

thermoplastics are heated above their

softening points, they can be cooled back

down to a different shape than where

they started. The most popular of these

is the polyimide (PI) film made from a

polyamic acid or ester resin. The reaction

creates closed rings along the chain as

shown in

Figure 2

.

T

This article was originally published in the Proceedings of the International Wafer-Level Packaging Conference 2018.

Figure 1:

Generic FOWLP construction (not to scale).

Figure 2:

Polyimide curing.