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Accelerated Testing of Flip Chip Underfill - Sensitivity to Aging/Moisture

Nikhil Vichare

Graduate Research AssociateDepartment of Systems Science and Industrial Engineering

State University of New York at Binghamton

Binghamton NY 13902

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Experiment

Performance in cycling is relative measure of degradation in preconditioning

Preconditioning Time, Temperature, Humidity • 72 hours - 6 months• Room temperature - 130oC • Dry N2 - 85%R.H.

• No Preconditioning

Subsequent Cycling in Liquid to Liquid Thermal Shock (LLTS)• 5 minutes hold at -55oC and 125oC• 10 seconds transfer time

Measure corner fillet cracking and delamination around bumps v/s number of LLTS cycles

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Experiment• 1056 die 4224 corner fillets

• 3 underfills Dexter FP4549 Namics U8437-3 Honeywell JM8802

• 2 fluxes Kester TSF-6522 Indium NC-SMQ 75

PB08 chips on 62 mil board with Taiyo PSR4000 mask

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Effects of Moisture

• It is well “established” that moisture exposure may weaken the underfill.

• Pressure Cooker Test (PCT) is a common accelerated test of sensitivity to humidity Also called unbiased HAST 85% R.H.,130°C, 2 atm.pressure (JESD22-A118)

• Is PCT meaningful?

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Fillet Cracking in Cycling after PCT

14-16 mil Thick Namics U8437-3 Fillets (Kester TSF-6522 Flux)

• Clear effect on subsequent cycling

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Num

ber

of F

illet

s

Crack No Crack Direct LLTS

72 Hours PCT + LLTS 144 Hours PCT + LLTS

LLTS Cycles

No Preconditioning

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PCT?• Characteristics of damage in cycling after PCT different from

that after other humidity/temperature exposure

85°C/85%R.H. 3 Weeks + LLTS PCT 72 Hours + LLTS

• Also discoloration of underfill and solder mask (see poster)

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Time/Temperature and/or Moisture has an Effect !

• Faster Cracking of Namics (TSF-6522) after 85°C/30% R.H.

14-16 mil Thick Namics U8437-3 Fillets (Kester TSF-6522)

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Num

ber

of

Fill

ets

Crack No Crack2 weeks

3 weeks

No Preconditioning1 week

LLTS Cycles

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More Moisture is Worse

2 Weeks of Preconditioning of 14-16 mil Thick Namics U8437-3 Fillets (Kester TSF-6522)

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Num

ber

of F

ille

ts

Crack No Crack

85°C/85%R.H.

85°C/30%R.H.

LLTS Cycles

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Also Faster Delamination from Solder Bumps after Moisture Exposure

250 1000 2000

1000 2000 3000

• After 3 Weeks of 85°C/85% R.H.

• Without Preconditioning

Namics U8437-3 (Kester TSF-6522)

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Acceleration?

Similar cracking in subsequent cycling for some

encapsulant/ flux/ fillet thickness combinations

Example

• 3 months aging in ambient 85°C/30%R.H. for 3

weeks

• 6 months aging in ambient 85°C/85%R.H. for 3

weeks

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Acceleration?• 3 months ambient 85°C/30%R.H. for 3 weeks• Namics(Kester) 14-16 mil fillets

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20

30

50

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50

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20

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25

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Num

ber

of F

illet

s

No Crack

Crack

85°C/30% R.H. 3 Weeks

3 Months Ambient

Namics(Kester)

LLTS Cycles

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Acceleration?

• 3 months ambient 85°C/30%R.H. for 3 weeks• Same material thinner fillets (10-12 mils)

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2500 500

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Num

ber

of F

illet

s

No Crack

Crack3 Months Ambient

85°C/30% R.H. 3 Weeks

LLTS Cycles

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Acceleration?• 3 months ambient 85°C/30%R.H. for 3 weeks• Dexter(Indium) 10-12 mil fillets

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20

30

500

1000

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2500 500

1000

1500

2000

2500

Num

ber

of F

illet

s

No Crack

Crack

3 Months Ambient

85°C/30% R.H. 3 Weeks

Dexter Indium

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14Namics U8437-3 (Kester TSF-6522) 14-16 mils

Acceleration?

• 6 months aging in ambient 85oC/85%R.H. for 3 weeks

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20

30

500

100

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Num

ber

of F

illet

s

Crack No Crack

6 Months Ambient 85°C/85%R.H. 3 weeks

LLTS Cycles

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Acceleration?

• 6 months ambient 85°C/85%R.H. for 3 weeks

and 3 months ambient 85°C/30%R.H. for 3 weeks

(totally different cracking in subsequent cycling) for

other encapsulant/ flux/ fillet thickness combinations

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Acceleration?• 3 months ambient 85°C/30%R.H. for 3 weeks• Honeywell (Kester) 12-14 mil fillets

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20

30

500

1000

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2500

500

1000

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2000

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Num

ber

of F

ille

ts

No Crack

Crack

3 Months Ambient

85°C-30% R.H. 3 week

LLTS Cycles

Honeywell (Kester) 12-14 mils

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Acceleration?

Dexter FP4549 (Indium NC-SMQ 75) 10-12 mils

• 6 months ambient 85°C/85%R.H. for 3 weeks• Dexter (Indium) 10-12 mil Thick Fillets

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Num

ber

of F

illet

s

NoCrack

Crack

6 Months Ambient

85°C-85% R.H. 3 week

LLTS Cycles

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Moisture isn't Everything!

• Underfill also weakens with time (even in dry storage)

• That’s why we didn't have a single acceleration

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Effect of Time As Well• Aging in Nitrogen Cabinet

10-12 mil Thick Namics U8437-3 (Kester TSF-6522)

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3000 500

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2000

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Num

ber

of F

illet

s

No Crack

Crack

No Preconditioning

3 Months N2

6 Months N2

LLTS Cycles

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1000 2000 3000

1000 2000 3000

• After 3 Months Aging in Nitrogen Cabinet

• Without Preconditioning

• Faster Delamination from Solder Bumps

Effect of Time As Well

Namics U8437-3 (Indium NC-SMQ 75)

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Preconditioning Sometimes Decreased Cracking

10-12 mil Thick Honeywell JM88202 (Kester TSF-6522)

• 85°C/30%R.H. reduces subsequent cracking of Honeywell (Kester)

• Effect of time/temperature or moisture?

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Num

ber

of F

illet

s

Crack No Crack

3 weeks

No Aging

1 week 2 weeks

LLTS Cycles

No Preconditioning

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More Moisture Increased Cracking

• At same temperature / time more moisture (85°C/85%R.H.) is worse in

this case

10-12 mil Thick Honeywell JM88202 (Kester TSF-6522)

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Num

ber

of F

illet

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Crack No Crack

LLTS Cycles

85°C/30%R.H. 2 Weeks

85°C/85%R.H. 2 Weeks 85°C/30%R.H.

3 Weeks

85°C/85%R.H. 3 Weeks

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Moisture Not Always Bad • A little moisture seems to help (same time /temperature)• Dexter (Indium) 8-10 mil fillets

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Nu

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of F

illet

s

NoCrack

Crack

3 Months Ambient

3 Months N2 Cabinet

LLTS Cycles

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SUMMARY

• Moisture and time/temperature each affect underfill

properties

• Time/temperature may not always be bad (depends on

encapsulant)

• Sometimes a little moisture reduces subsequent cracking

• Relative contribution of time/temperature Vs moisture

depends on encapsulant/ flux combination

• Design/choice of accelerated test must account for this