Via-In-Pad

This project explores whether PWB laser drilled micro-vias positioned in surface mount lands used for attachment of ball grid array components, have a negative impact on long term product reliability. Both plastic and ceramic ball grid arrays are included in the study.

Project stage: 
Project type: 

Idea Information

Background: 

Previous studies have indicated that, although the source of voids in ball grid array solder joints, laser drilled microvias have little impact on the performance of BGA interconnect reliability in accelerated thermal cycling (ATC). It has also been noted that SnPb ball grid array solder joints predominantly fail  on the component substrate side and not at the ball to PWB interface, typically where the micro via induced void is PWB or component substrate interfaces. This raises the question of when failures do occur at the PWB side does the presence of a void accelerate this failure.

 

Fig 1. Void in BGA solder joint caused by outgassing of a microvia positioned in the centre of the attachment pad

 

 

 

Problem: 

A current practice to mitigate against void related reliability issues is to fill the micro-via and plate over it eliminating any voiding. This however, adds cost to the PWB fabrication process. We therefore need to understand whether micro via induced voids do reduce cycles to failure in ATC testing

Definition Information

Goals / Benefits: 
Determine if void caused by microvia in pad has an effect on Pb-free solder joint reliability in BGA’s and QFN’s
High level objectives: 
  • Design test vehicle to compare via in pad vs. no via in pad on same board.
  • Select range of BGA and QFN components
  • Perform accelerated thermal cycling on test vehicles
  • Conduct Weibull analysis of results and investigate failures using cross sectional and void percentage analysis

 

Approach: 

Test Board

  • 4 each of 4 component types per board
  • 2 sets on left – Via in Pad
  • 2 sets on right – No vias in pad
  • Test Board Size: 6.5” x 7”x .125” thick
  • Connector not populated – for wiring only
  • Extra ground lug point added

 

 

 

  • 1mm pitch CBGA 483, ball size 0.89mm diameter
Same component used on Multiple Alloy Screening project, SAC Aging project, and SAC Acceleration Factors Project
From NTK (with permission from Freescale)
SAC405 solder balls added at Premier Semiconductor
 
  • 0.8mm pitch 192 CABGA
From Practical Components
Procured with “large” die (special order code) Die size 475x475 mils
Ball size: 0.46mm, SAC305
 
  • 0.5mm pitch 84 CTBGA
From Practical Components
Procured with “large” die (special order code) Die size 200x200 mils
Ball size: 0.3mm, SAC305
 
  • 10x10mm 72 I/O QFN (MLF)

-From Practical Components

 

 

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