Countermeasure for Whisker …Analysis on Sn system "whisker"(Extraction from our Technical Report)

About "whisker"

It is known that the whisker is the phenomena in which needle-shaped or nodule-shaped single crystal metal grows spontaneously over metal surface when processing Sn and Zn Plating.
The problems of this phenomena are such as occurrence of electric short-circuit caused from the growth of whiskers in electronic circuit. Although various causes are thought to be their factors such as "Phenomena of intermetallic compounds or diffusion" "Phenomena of Galvanic corrosion" "Phenomena of external stress" "Phenomena of difference from heat expansion coefficient", their fundamental mechanism have not yet been clarified.
The method of Whisker test (example) and Whisker generating example which we are now grappling with are as follows.
 
Fig.1:Whisker Evaluation Method Table (example)
  Test Purpose Test Condition and Time
Room temperature test
Observation of growing whiskers primarily occurred by phenomena of intermetallic compounds or diffusion. Left in room temperature 5000h
High temperature and high humidity test
Observation of growing whiskers primarily occurred by Phenomena of Galvanic corrosion. 55 degree -85%RH 5000h
Temperature cycle test
Observation of growing whiskers primarily occurred by phenomena of difference of heat expansion coefficient. -40°C⇔85°C(each maintainning temperature10 to 30min) 1500cycle
-55°C⇔85°C(each maintaining temperature10 to 30min) 1500cycle
External stress test
Observation of growing whiskers primarily occurred by phenomena of external stress. (1) Connector connection test(connection test with real products)
(2)Load test…fixed load (300gf) maintaining 500h with using spherical0.1φzirconia
 
Fig.2:Whisker generating example
Whisker that occurred under the room temperature test
Whisker that occurred under the high temperature and high humidity test
Whisker that occurred under the temperature cycle test
Whisker that occurred under the external stress test
 
Whisker that occurred under the room temperature test
Whisker that occurred under the high temperature and high humid test
Whisker that occurred under the temperature cycle test
Whisker that occurred under the external stress test
 
 
"Whisker Control Method"
We display here the example of analysis which aim principally at growth of intermetallic compounds as our own grapple of control for Sn whisker.
 
 
 

Proposal Example (1)
Examination for Undercoat

There are great differences in growing or diffusion speed of intermetallic compounds when alter the kind of undercoat (Ni or Cu Plating in general).
The Fig. 3 shows the SEM observation photos of etching dispositioned pure Sn plating layer which was left in room temperature 20000h after Sn plating over Ni and Cu undercoat is completed.
We can observe Cu6Sn5 compounds grow in the form of “nodule-shaped” along Sn grain boundary in case of Cu undercoat, while“thin-foil-shaped”compounds grow in case of Ni undercoat.
We conjecture that the effect of whisker control which we can get from Ni undercoat is to mitigate the internal stress in pure Sn plating layer by growing of thin-foil-shaped Ni3Sn4 compound layer.
 
Fig.3: The Difference of Sn Compound Growth in Variety of Undercoat
Etched Sn plating over Ni undercoat
Etched Sn plating over Cu undercoat
 
Etched Sn plating over Ni undercoat
Etched Sn plating over Cu undercoat
 
 
 
 

Proposal Example(2)
Examination of Alloy Plating

The method which adds the second metal into Sn plating film is also considered to be an effective method for improvement of solder wettability.
Fig. 4 shows the consequence of the growing observation of etching dispositioned compound layer which was plated with various Sn system over C1100 and left in room temperature 20000h.
 
Fig.4:Difference of Sn compound growth which causes from the difference of Sn system plating finish
Pure Sn Plating over C1100
Sn-Cu Plating over C1100
 
Pure Sn Plating over C1100
Sn-Cu Plating over C1100
 
 
Sn-Bi Plating over C1100
Sn-Pb Plating over C1100
 
Sn-Bi Plating over C1100
Sn-Pb Plating over C1100
 
There are great differences on growth of Sn compound by adding second metal. The photos show the state of grain diameter of nodule-shaped compound getting smaller in comparison with Sn plating in case of Sn-Pb plating and Sn-Bi plating, while Sn-Cu plating shows the growth not in nodule-shaped but in pillar-shaped. The result of each alloy plating showed us the better whisker control effect than Sn plating (no-heat-treatment) after our room temperature test.
And moreover, it is known that the creating condition of Sn system compound or conduct of diffusion by combining undercoat shown in Fig.3, and the more effective whisker control will be expected. (under investigation)
 
 
 

Proposal Example(3)
Examination of Post-treatment after Plating

We’re already operating mass production system in partial products of adding heat-treatment (anneal treatment) on finishing plating.
Fig. 5 shows the result of observed growth of etching processed compound layer which both in post-treated and not post-treated after left in room temperature 20000h.
As the result shows the growth of Cu6Sn5 compound is controlled by heat-treatment.
 
Fig.5:The Difference of Sn Compound Growth in Heat-treated or Not
Pure Sn plating over C1100 without heat-treatment
Pure Sn plating over C1100
 
Pure Sn plating over C1100 without heat-treatment
Pure Sn plating over C1100
 
 
We found that we could control the growth of intermetallic compound through improvement process of "undercoat" "finish plating" "post treatment" in this article. And the whisker control effect under partial environmental test we could perceive. We are proposing this whisker control method utilizing each character with this analysis results and now they tied up with stable mass production achievement to our customers. But "whisker occurred from corrosion", "whisker occurred from the difference of heat expansion factor"and "whisker occurred from external stress" have difference in occurrence mechanism, so they need different aproach and improvement.
 
 
We are now grappling with evaluation and analysis to result in development from various poin of view aiming at "Zero Whisker Occurrence"under the all of the environmental test.