How to dramatically reduce BTC voiding

Posted 06/07/2013 by cderama

 

QFN packages

Typical BTC QFN package

 

 Reduced Voiding With Preforms

Acceptable levels of voiding vary by customer and application.  Typically, the tolerance for voiding is related to the expected installed life of the product.  While specific integrated circuits can have varying requirements, 25% voiding by total area is considered marginal.  Many customers target lower levels, with the understanding that a small percentage of outliers may reach 25%. 

Typically, if voiding exceeds the threshold (customer specific), rework is required.  The majority of customers do not reuse the IC, thus there is a tangible cost for excessive voiding.  Locally resoldering the reworked site also does not guarantee the voiding level, which also can result in PCB damage. 

One solution for the reduction of voiding is the use of flux coated solder preforms.  Available in tape and reel packaging, these preforms are placed on the PCB thermal pad prior to component placement.  The preforms are thin discs, resulting in a round preform on a square pad.  In the exposed pad area, solder paste is printed to corral the preform, and prevent it from shifting during assembly, prior to it being “sealed” when the component is placed.  The preform and solder paste do not overlap, but are adjacent. 

 

Flux Coated Preform

Flux coated preform surrounded by printed paste deposits

 

Alpha recommends that the preform contribute at least 80% of the total solder volume.  This has shown to provide voiding levels of between 8-18%, and the result can be optimized with reflow profile adjustments.  Two additional benefits of the preform solution are 1) eliminates outliers, which are cases of very high voiding, and 2)  the number of large voids is also reduced. 

Preform Box Plot

Boxplot chart shows flux coated preform solution (GREEN) has the lowest level of voids, and no cases of excessive voiding that would require rework.   Also examined with solid pad printed paste (PURPLE), 3 x 3 paste printed pattern with a non-flux coated preform (RED), a 2 x 2 paste printed pattern with a non-flux coated preform (YELLOW), and a 2 x 2 paste printed pattern – paste only (BLUE). 

Alpha is working with a limited number of beta site customers to demonstrate the flux coated preform solution.  Later in 2013, the manufacturing equipment needed to produce flux coated preforms for PCB void reduction applications will come online, supporting customers using stencil thicknesses of 3 mils to 6 mils.  Stay tuned!

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