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29

Chip Scale Review September • October • 2018

[ChipScaleReview.com]

The clouded view of WCSP inspection strategies

By M. Todd Wyant

[Texas Instruments]

emiconductor parts continue

to shrink at alarming rates.

As par ts approach g reater

than one million units on a three hundred

millimeter (mm) analog wafer, many visual

containment actions are being pushed past

the breaking point. System improvements

for detection are limited as algorithms, and

detection strategies are still limited when it

comes to small defects on traditional flows.

Figure 1

shows the progression from 1mm

part sizes down to 0.125mm, highlighting

the difficulty encountered when trying to

detect defects on such small components.

Today, more than ever, there is an

increased pressure to ship parts without

defects and achieve single-digit defective

parts per million (dppm) levels. The

continued industry push is to apply new

technology and methods to ultimately

create a situation where all defects are

contained or eliminated prior to shipment.

Today’s inspection equipment can scan

par ts quick ly and produce det ailed

inspection reports to drive improvements,

but with part sizes exponentially shrinking

and key return defect modes not shifting,

the push now is to f ur ther improve

inspection capabilities to detect problems

even with the smallest of components.

The cont i nued pu sh t o i nc r e a s e

resolution has reached the tipping point.

Resolution is becoming so detailed that all

components are starting to look different.

The variation being introduced with this

high resolution is leading to false rejectivity

that is hindering the situation and time

needed to drive improvements (

Figure 2

).

Implementing filtering and other methods

to prevent false rejectivity is leading back

to release of the same returned items into

the pack solutions that ultimately end up

back into the customer’s hands. In addition,

this complexity drives engineering time

and effort that ultimately impacts cost.

This balance between over-rejection

and rejected shipments has been a long-

standing problem, and demonstrates the

real challenge of implementing a quality

assu rance system. Meeting moder n

manufacturing demands of delivering

fast and accurately is challenging, while

simultaneously driving cost through scrap

reductions and improved process efficiency.

Current working path

Today, assembly sites are driving to

implement inspection solutions that use

infrared (IR) or standard high-resolution

imaging to detect, measure, and contain

defects. These methods rely heavily on

lighting, imaging and software to detect and

contain rejectivity. However, the issue with

this method has shown that false rejectivity

will be a way of life. During testing on

many different platforms, anywhere from

2-8% of false rejectivity has occurred

with the best case settings. Depending

on the system type, productivity loss can

be anywhere from 5-50% for increased

inspection time required.

T h e n e w m e t h o d s h a v e a l s o

demonstrated limitations in finding and

detecting all rejectivity. In addition, as

shown in

Figure 3

, the use of rotated silicon

structures prevent IR from passing through

the substrate during visual inspection. In

the system during inspection, the silicon

lattice prevents clean exit of the infrared

light penetration through the component.

The result of IR on these products is a black

surface that is not visible with detection

software. The chart shows the difference

with varying substrate rotation, with and

without backside coatings below, and

highlights the challenges of inspection

using IR technology solutions.

Challenging defects are really what is

driving inspection capability increases,

while cost and end users continue to drive

parts smaller.

Figure 4

shows the dilemma

with any inspection process as defect sizes

are very small. Cracks and damage cross

S

Figure 1:

This graphic shows the progression from

1mm-part sizes down to 0.125mm, highlighting the

difficulty encountered when trying to detect defects

on such small components.

Figure 2:

Examples of challenging rejects: a) Side

wall crack; b) Corner crack; c) Common edge chip; and

d) Hairline cracks.

Figure 3:

Rotated silicon effects on IR transmissivity.

Figure 4:

Examples of marginal defects: a) Whisker crack; b) Corner hairline crack; and c) Crack failure limit.