Countermeasure for Void  Analysis on Void occurrence phenomena with"Sn plating-Ag paste"(Extraction from our Technical Report)

Purpose of Examination

After the investigation of time-deterioration of Sn system plating film and conductivity adhesive (afterwards referred to as Ag paste) in long term reliability evaluation of SMD parts, we trace the cause of occurrence and propose alternative plan of Kirkendall Void (afterwards referred to as Void).
 

Method of Examination

After spreadding Ag paste over Sn film, harden disposition is carried out in 150°C‐30 min. After the heat resistence (175°C) test and high temperature and high humid (85°C×85%RH) test and we observe the surface state by SEM after eliminating Ag paste.
And measure the electric features of “plating-Ag paste”after accelerating test for each.
 

Results of Examination

Heat Resistence (175°C) Test
The roughness occurred on Sn surface in initial stage. The passing of heat resistence time, they grow like pinhole form and occur Void by piling up of them. They grow to enormous Void in the end while disapearing pure Sn plating layer.
 
Heat Resistence (175°C) Test01
Heat Resistence (175°C) Test02
Heat Resistence (175°C) Test03
Heat Resistence (175°C) Test04
 
Fig.1: Photos of Surface Condition of Eliminated Part of Ag Paste after Heat Resistence Test
 
 
High Temperature and High Humidity (85°C×85RH)Test
The roughness has occurred on Sn surface in initial stage. The reaction progresses like dissolving over the surface of Sn.
The dissolving corrosion progresses from the center of Ag pasted part, and pure Sn layer will disappear in the end. The corrosion reaches the undercoat plating to depth direction.
 
High Temperature and High Humidity (85°C×85RH)Test01
High Temperature and High Humidity (85°C×85RH)Test02
High Temperature and High Humidity (85°C×85RH)Test03
High Temperature and High Humidity (85°C×85RH)Test04
 
Fig.2: Photos of Surface Condition of Eliminated Ag Paste Part after High Temperature and High Humid Test
 
 
The difference was shown in Cl peak when we compared with the ingredient of 0h and 85°C×85%RH -500h. Cl peak which centered in corrosion part was detected after 500h leave. We conjecture that the corrosion is accelerating by Cl which is included in Paste.
 
Fig3 : Result from mapping analysis
 
Result from mapping analysis01
Result from mapping analysis02
 
Result from mapping analysis03
Result from mapping analysis04
 
 

Electric Features

We measure the electric features in "plating-Ag paste" after the heat resistence test and high temperature and high humid test.
As the Voids which occurred from heat resistence test are generated extensively, the rise of contact resistance is remarkable (approx.50mΩafter 168h).
High temperature and high humidity test shows us the less deterioration than in heat resistence test as the corrosion progresses from the center part.
 Result from contact resistance measurement of Sn plating after various environmental test.
 
Fig4: Result from contact resistance measurement of Sn plating after various environmental test.
 

Proposal of Alternative Plating

We could confirm the time-varying of the "Void" occurrence, so that we proposed alternative plan in this matter. We could achieve the better consequence in alternative plating in respect of stability of contact resistance value after heat resistence test and high temperature and high humidity test.
Result from contact resistance measurement of alternative plating after various environmental test.
 
Fig5: Result from contact resistance measurement of alternative plating after various environmental test.