Die Attach Process Development Using B-stageable Epoxies Elgin F Bravo May 11th 2001 Advisors Dr. R. Chung Randall Walberg SJSU NSC

Motivation Reduce package size –Ability to wirebond close to edge of die Easy to die attach small dies –Dies smaller than 1mm 2 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 1mm

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 %TRANSMITTANCE%TRANSMITTANCE Functional group

Chemical Reaction in Epoxies O || R 1 - C = C - R 2 + R 3 - C - O - O - H ---> Alkene Peroxyacid O H / \.. | R 1 - C - C - R 2 + H - N - R 3 ---> R 1 - C - CH 2 - R 2 | | H H 2 N - R 3 Epoxide Amine Polymer chain (hardener)

Components in B-stageable Epoxies

Bond Structure of Epoxide Molecules BisphenolA/ Epichlorohydrin epoxy Cresol novolac epoxy CH2 CH

Infrared Transmittance as function of Curing Time Sumitomo 4291 Amine (N-H) Carboxyl (COOH) Epoxide C-C

Infrared Transmittance as function of Curing Time Amine (N-H) Carboxyl (COOH) Epoxide C-C

Curing time & temperature

Moisture Sensitive Levels Level 1: 168 hrs 85  C / 85% Relative Humidity (Excellent) Level 2: 168 hrs 85  C / 60% Relative Humidity (Good) Level 2A: 150 hrs 60  C / 60% Relative Humidity (Satisfactory) Parts go through reflow oven reaching a max temperature of 235  C Packages assembled onto PCB are reflowed at ~ 220  C

Top sonoscan view of a Package Die Leads Cu substrate

Sonoscan Results MSL 1 Front viewBack view

Sonoscan Results MSL 2 Front viewBack view

Sonoscan Results MSL 2A Front viewBack 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