Whisker Generation Mechanism of Lead-free Soldered Joints

Transcription

1 Whisker Generation Mechanism of Lead-free Soldered Joints Mutsumi YOSHINO Masaki SANJI Syuntaro IGURO Due to the environmental restriction of SOC (Substances of Concern) in Europe, the lead-free solder for electronic devices, especially for consumer products, has been undergoing vigorous development in Japan. Also for automobiles, the ELV instruction was approved in Europe and restrictions on the substances Pb, Cr6+, Hg and Cd, were implemented in 3. At present, the Sn-Pb solder for electronic contacts is excluded from these restrictions, but in the near future, Sn-Pb solder will be included in these restrictions. As is well known, automobiles operate in a severe environment, i.e. high temperature and high humidity, therefore the application of lead free solder faces a lot of issues. One of the most important issues is that of whiskers. Usually, it is said that whiskers can be prevented by melting the solder to release the stress. However we discovered whiskers in naturally solid solders. Consequently, we examined some tests to study the mechanism of whisker generation. In this paper, we introduce the results of these tests and the whisker generation mechanism. Key words: Whisker, Lead-free solder, ELV instruction, SOC (Substances of Concern)

2 Photo 1 Out-view of whisker on PCB Fig. 1 Results of EPMA analysis

3 Whisker Photo 2 Out-view of whisker Time (h) (a) Low activated flux Time (h) (b) Activated flux for Pb-free soldering Fig. 2 Whisker length change with the lapse of time 3 1 ~1 11~ 21~3 31~ 41~5 51~ 61~7 71~ 81~9 91~ Fig. 3 Distribution of whisker length appeared from ref.

4 3 1 ~1 11~ 21~3 31~ 41~5 51~ 61~7 71~ 81~9 91~ Fig. 4 Sn-3.Ag-.5Cu-.3Sb ~1 11~ 21~3 31~ 41~5 51~ 61~7 71~ 81~9 91~ 3 1 ~1 11~ 21~3 31~ 41~5 51~ 61~7 71~ Fig. 5 Sn-4.Ag-.5Cu 81~9 91~ 3 1 ~1 11~ 21~3 Fig. 8 Sn-Cu-Ni 31~ 41~5 51~ 61~7 71~ 81~9 91~ Fig. 6 Sn-3.Ag-.5Cu-.4Ni Fig. 7 Sn-3.Ag-.5Cu-.4Ni-.3P ~1 11~ 21~3 31~ 41~5 51~ 61~7 71~ 81~9 91~ ~1 11~ 21~3 31~ 41~5 51~ 61~7 71~ 81~9 91~ Fig. 9 Sn-Pb eutectic

5 Number Max. length (µm) A B C D Maximum whisker length ~1 11~ 21~3 31~ 41~5 51~ 61~7 71~ 81~9 Whisker length (µm) 91~ 11~11 111~1 121~13 Fig. 1 Difference from PCB suppliers (a) No conformal coated ~1 11~ 21~3 31~ 41~5 51~ 61~7 71~ 81~9 91~ Whisker length (µm) (b) Conformal coated Fig. 11 Whisker distribution after cleaning

6 C% C95% C95% 85 C95% Maximum length (µm) 1 3 Amount of water (g/m 3 ) (a) 85 C95% C95% C% C95% 3 Amount of water (g/m 3 ) (b) Maximum length of whisker Fig. 12 Difference of test condition Solder Cu-pad (a) Whiskers exist Solder Cu-pad (b) No whiskers exist Fig. 13 Difference on the cross-section of soldered pad Local-battery corrosion Non-corrosive Whisker Fig. 14 Whisker growth processes

7 Corrosion rate (%)=(L 1 + L 2 )/ L L 1 L L2 W = 2Dt D = Aexp Q kt exp ( B ) RH ( C B T + RH) ( C B T + RH) Corrosion rate (%) Fig. 15 Cross-section of Cu-circuit edge : Mean of measured data Test time (h) Corrosion rate (%) : Mean of measured data : Calculated value Test time (h) Fig. 17 Corrosion ratio change with the lapse of time Fig.16 Corrosion ratio change with the lapse of time

8 Corrosion length (µm) C95%RH C95%RH C95%RH C%RH Corrosion length h (C/T + B/RH) Fig. 18 Corrosion length vs. number of whisker under several corrosion parameter Solder Cu-pad Corrosion rate=% Noncorrosive Solder of Cu-pad side (Sn) Corrosion rate=% Noncorrosive Corrosion rate=% Fig. 19 Analysis model (SnO) (SnO) Noncorrosive Non-corrosive Corrosion rate=% Noncorrosive Comperession stress (MPa) Corrosion rate (%) Fig. Compression stress under several corrosion ratio 1 5 Time (h) Fig. 21 Number of wisker under several corrosion ratio

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