Probing Lead Free Solder Bumps in Final Wafer Test

Transcription

1 Probing Lead Free Solder Bumps in Final Wafer Test By Sam McKnight Southwest Test Workshop June 2002 Microelectronics

2 Outline Why lead free? Background Info Focus areas Setup Lead/Tin and Tin Rich results Summary Acknowledgements 2

3 Why Lead Free? Increasing use of integrated circuits and shortened product life cycles prompted a world wide environmental concern regarding the disposal of electronic components. Lead countries with environmental initiatives are Japan, Europe and the US All levels of packaging including the interconnect to the integrated circuit are being addressed. This presentation focuses on wafer level only 3

4 Background Info Cobra probe (5 mil diameter - flat tipped) is the primary contacting system for C4 VLSI test C4 = Controlled Collapse Chip Connection (Solder Bumps) C4's are normally reflowed before and after test Tests temperatures -10, 25, 85 and 100C Thermal unit range is C probe space transformer limit 100C (self imposed) 4

5 Focus areas Baseline comparison measurements to Lead Tin Contact Resistance Cleaning Material Transfer (Pickup) To Tip Pad Deformation Alignment to C4 s (Solder Bumps) 5

6 layout 76 contacts total Die size ~ 9.1 mm square d pads 6

7 Test wafer build structure Lead-Tin/Lead Free ~4.5 mils 4.5 mils Unreflowed C4's (photo resist removed) BLM (Ball Limiting Metallurgy) Insulator Si wafer Scale none 7

8 Setup TEL P8XL wafer probing system with integral -10 to 140C thermal unit w/ IBM custom tester interface with hand wired space transformer Z drive - maximum impact velocity (~40 mm/sec) SARA (Socket Analog Resistance Analyzer) (DC switching matrix with measurement unit) Cres Measurement method - "pin return" with 10 ma forcing current and 5V clamp w/nulling file - to subtract path resistance Cleaning frequency - 1/wafer (>300 sites) - 5 micron abrasive material Environment - mini-clean hood over prober 8

9 Contact Resistance Measurement VI source ~ 10 ma@ 5V Contact under test Return path Cobra contact C4's Note: actual return path pins ~ 75 Wafer 9

10 Cobra Measurements 50 force sample measurements outer row - Ceramic lower guide plate Amb Temp - after ~ 11K touchdowns, ~10 Temperature cycles PCB temperature vs overdrive Force (in grams) Pin 1 Pin 2 Pin 3 Pin 4 Pin 5 Pin 6 Pin 7 Pin 8 Pin 9 Pin 10 PCB deflection (in mils) C 25C 85C 100C deflection (mils) Overdrive setting (in mils) 10

11 Un Reflowed C4 s Lead free Lead Tin 11

12 Contact Resistance Lead Tin C4 s 10 Average Contact resistance PbSn 97/3 UNREFLOWED C4's Each plot point = 86K measurements (OHMS) C 25C 85C 100C mils 7 mils 8 mils 9 mils Contact + Overdrive 12

13 Contact Resistance - Lead Free C4 s Note scale Average Contact resistance Lead Free Each plot point = 86K measurements C 25C 85C 100C (OHMS) open readings (500 ohms) 0.2 Sample contaminant open pin mils 7 mils 8 mils 9 mils Contact + Overdrive 13

14 -10 C Cobra/C4 Photos 6 MIL O D 7 MIL O D 8 MIL O D 9 MIL O D 6 MIL O D 7 MIL O D 8 MIL O D 9 MIL O D 14

15 25 C Cobra/C4 Photos 6 MIL O D 7 MIL O D 88 MIL O D 9 MIL O D C4 Pad 6 MIL O D C4 Pad 7 MIL O D C4 Pad 88 MIL MIL O D C4 Pad 9 MIL O D 15

16 85º C Cobra/C4 Photo s 6 MIL O D 7 MIL O D 8 MIL O D 9 MIL O D C4 Pad C4 Pad C4 Pad C4 Pad 6 MIL O D 7 MIL O D 8 MIL O D 9 MIL O D 16

17 100 C Cobra/C4 Photo s C4 Pad C4 Pad C4 Pad C4 Pad 17

18 Pad Deformation C4 AVERAGE deformation comparisons PbSn to Sn Deformation 2 Inconclusive!! 1.5 O ffset probe Unprobed Centered probe (MILS) PbSn-10C PbSn25C PbSn85C PbSn100C Sn-10C Sn25C Sn85C Sn100C Test temperature S. McKnight B. Buddington 04/10/01 18

19 Typical Wafer level Contact Resistance plot Lead Free C4 s Sn Average Contact Resistance/site 6 mil OD - temperature -10C - 2nd pass 283 sites/wafer Note scale Cres (ohms) Finish h Notch Start S. McKnight B.Buddington 04/04/01 19

20 Typical Wafer level Contact Resistance plot for Lead/Tin C4 s PbSn Baseline Average Contact Resistance/site 6 milod - temperature - 25C 283 sites/wafer Note scale Finish Notch Start S. McKnight B.Buddington 03/29/01 20

21 Summary Contact resistance for Lead Free << 1-2 Ohms (temperature independent) Pad deformation inconclusive not an issue for post reflow operation r issues - auto alignment (non production wafers) Partial radius on top of pad not an issue Cleaning frequency, 1/wafer (~300+Die) is anticipated using standard (abrasive) cleaning materials Material transfer to probe tip comparable to Lead/Tin 21

22 Acknowledgements Barry Buddington IBM - software/setup operations Gobinda Das IBM PhD Steve Kilpatrick IBM PhD Amador Cantu Jr. TEL Field Service 22