Die Attach Process Development Using B-stageable Epoxies

Elgin F BravoMay 11th 2001

AdvisorsDr. R. Chung Randall WalbergSJSU NSC

Motivation

• Reduce package size– Ability to wirebond close to edge of die

• Easy to die attach small dies– Dies smaller than 1mm2

• Reduce cycle time– Increase profits

• Innovative

– New to industry

Outline• Objectives• Last semester accomplishments• Background on B-Stage epoxies• Literature review• Percent conversion • Infrared Absorption Spectroscopy• Moisture Sensitivity Level • MSL results• Conclusion• Future work

Objectives

• Determine process feasibility and ease of manufacturing– Coating of die with b-stage epoxy– Die attach die onto leadframe– Elimination of epoxy bleed

• Study moisture and heat sensitivity of B-stageable epoxies when used as die attach materials in leadless leadframe packages.

Last Semester Accomplishments

B-Stage epoxy Standard epoxy

1mm1mm

Background on B-stageable Epoxies

• A-stage --> unpolymerized --> 0% conversion

• B-stage --> some polymerization --> 28 ~ 45 % conversion

• C-stage --> mostly polymerized --> 90% conversion

Literature Review

• Percent conversion determines bond reliability

• – Higher degree of conversion lower bond

reliability

– Lower degree of conversion higher bond reliability

Determining % Conversion

• Differential Scanning Calorimetry (DSC)– dX/dt = K(T) x f(X)

» k(T) reaction rate constant» X fractional conversion» t cure time

• Infrared Absorption Spectroscopy– Epoxide + Hardener = polymer chain

Infrared Absorption Spectroscopy

E = h

= c/

NaCl disk, 2.5 mm thick

Thin layer of epoxy ~ 0.5mm

Detector

= 2.5e-4 to 25e-4 cm

Infrared Spectroscopy

%

T

R

A

N

S

M

I

T

T

A

N

C

E

Functional group

Chemical Reaction in Epoxies O || R1 - C = C - R2 + R3 - C - O - O - H --->

Alkene Peroxyacid

O H / \ . . |

• R1 - C - C - R2 + H - N - R3 ---> R1 - C - CH2 - R2

| | H H2N - R3

Epoxide Amine Polymer chain (hardener)

Components in B-stageable Epoxies

Sumitomo CRP-X4291Component by weight (%)Filler (silica) 35 - 65Epoxy resin 25 - 35Solvent 10 - 20Hardener (Aromatic diamine) 5 - 12

Ablebond 8006Component Composition (%)Ethylene glycol monobutyl 20 - 35 ether acetateEpoxy cresol novolac 15 - 30Bisphenol A/ 5 - 20 Epichlorohydrin based epoxy resinEpoxy resin 1 - 10Aryl amine 5 - 10

Bond Structure of Epoxide Molecules

BisphenolA/ Epichlorohydrin epoxy

Cresol novolac epoxy

CH2

CH2CH

Infrared Transmittance as function of Curing TimeSumitomo 4291

0

10

20

30

40

50

100015002000250030003500

wavenumber (1/cm)

% T

ran

sm

itta

nc

e

0 min 20 min 40 min 60 min

Amine (N-H)

Carboxyl (COOH)

Epoxide C-C

Infrared Transmittance as function of Curing Time

0

20

40

60

80

100

100015002000250030003500

wavenumber (1/cm)

% T

ran

sm

itta

nce

0 min 20 min 40 min 60 min

Amine (N-H)

Carboxyl (COOH)

Epoxide C-C

Curing time & temperature

B-STAGE 1 HR @ 120oC

MIN SUBSTRATE TEMPERATURE 150oC

FINAL CURE 1 HR @ 150oC

B-STAGE 1 HR @ 100oC

MIN SUBSTRATE TEMPERATURE 150oC

FINAL CURE 2 HRS @ 160oC

Sumitomo CRP-X4291

Ablebond 8006

Moisture Sensitive Levels

• Level 1: 168 hrs 85C / 85% Relative Humidity (Excellent)• Level 2: 168 hrs 85C / 60% Relative Humidity (Good)• Level 2A: 150 hrs 60C / 60% Relative Humidity

(Satisfactory)

• Parts go through reflow oven reaching a max temperature of 235C

• Packages assembled onto PCB are reflowed at ~ 220C

Top sonoscan view of a Package

Die

Leads

Cu substrate

Sonoscan Results MSL 1

Front view Back view

Sonoscan Results MSL 2

Front view Back view

Sonoscan Results MSL 2A

Front view Back view

Conclusion

• Process was concluded to be feasible – Die coating– Die attach– Elimination of epoxy bleeding

• Infrared absorption spectroscopy might not be reliable for this type of study

• Both B-stageable epoxies fail all three Moisture Sensitivity Levels

Future Work

• Work together with suppliers to improve MSL rating to at least MSL 2

• Thermal modeling– Determine how well epoxy dissipates heat

• Electrical testing– Determine electrical conductivity of epoxy

Acknowledgements

• Dr. Richard Chung

• Randall Walberg

• Dr. Roger Terrell

• National Semiconductor