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22

Chip Scale Review July • August • 2019

[ChipScaleReview.com]

mater ials and develops pat ter ns i n

some laye r s, and t he ave r age CTE

o f t h e s em i c o n d u c t o r l a y e r i s a

constantly evolving parameter. Many

of the FO processes also happen at

eleva t e d t emp e r a t u r e s: poly imide

( PI ) c u r i ng , EMC mol d i ng , EMC

curing, etc. Given the evolving nature

of average CTE, there is no perfect

match for a carrier, and a carrier CTE

ne ed s t o be chos en t o cont r ol t he

warp throughout the FO process to be

smaller than a certain threshold.

How the other carrier characteristics

i mp a c t wa r p i s emb e d d e d i n t h e

formula (

Eq. 1

). To more clearly see

the relationship, we can make some

further simplifications. If we assume

the semiconductor layer is significantly

thinner than the car rier, the second

and third terms in the denominator of

the war p formula can be ignored in

comparison to the first term, and the

warp expression becomes:

He r e we c a n s e e t he a dd it ion a l

trends clearly. Under the conditions

stated above, the war p is inversely

proportional to the carrier’s Young’s

modu l u s ( YM). T he wa r p i s a l s o

inversely proportional to the square

of t h e c a r r i e r t h i ck n e s s . I nd e e d ,

when we use the t ypical conditions

encountered in FO and plotted out the

warp as a function of carrier YM and

carrier thickness, we see this trend in

Figures 2-4

.

Figure 5a

combines the effect of

ΔCTE and YM in a 3D plot. It clearly

shows the linear relation to ΔCTE;

and for any ΔCTE, it also shows that

a higher Young’s modulus can reduce

warp effectively.

Fi gure 5b

shows iso -wa r p l i ne s

on a ΔCTE vs. warp plot. The purple

triangle is meant to illustrate how YM

can impact ΔCTE tolerance. When

maximum allowable war p is 250µm

in this example, a glass carrier with

60GPa YM can only tolerate a ΔCTE of

~0.35ppm/°C. If one were to engineer a

glass carrier with 140GPa YM, tolerable

ΔCTE would increase to ~0.8ppm/°C.

Such scenarios could be needed as future

FO packages involve more layers and a

broader materials set, and glass has the

potential to meet such high YM values.

Figure 2:

Warpage as a function of glass thickness for: a) L=300mm, and b) L=500mm.

Figure 3:

Warpage as a function of the glass Young’s modulus for: a) L=300mm, and b) L=500mm.

Figure 4:

Warpage as a function of ΔCTE(ppm/ºC) for: a) L=300mm, and b) L=500mm.

Figure 5:

a) Effect of ΔCTE and Young’s modulus on warpage for L=300mm; b) The purple triangle illustrates

how Young’s modulus can impact ΔCTE. A higher Young’s modulus helps relax the CTE match requirement.

Eq. 2