Polymer Solder Balls
17 December 2007
The use of plastic balls that are metallised and fixed to the base of Ball Grid Array (BGA) and Chip Scale Package (CSP) packages is not new and have been circulated in the industry for the last five or six years.
Technical papers on the use and reliability of these terminations have been presented at IMAPS, Pan Pacific and IEEE conferences. Basically the solder balls consist of a polymer core coated with a copper Cu layer of around 5-8um and then covered with 18-20um of eutectic and/or high melting tin/lead solder. Spheres are also available with lead-free surfaces to make them compatible with current soldering technology and legislation, an example may be Sn/Ag3.5.
The example image shows a row of terminations after reflow soldering, to the casual observer they look not much different to conventional ball grid array joints. However if you look closely at the surface they have a very smooth surface and depending on the wetting there may be no demarcation line between the ball and the solder coating. It is difficult to see if the solder has wetted part or over the full surface of the balls. This type of package can be assembled with the traditional paste print and reflow. As there is a very limited volume of solder on the surface of the ball and the ball does not reflow and collapse during soldering, flux or flux gel would not be satisfactory. The balls on the CSP were produced by Sekisui Chemical of Japan, the material is made from divinylbenzene.
The second image shows an X-ray of the ball which apparently is hollow, although in fact this is not the case, it is simply the difference in density between the materials being viewed. The solder joints formed and the outer coating of copper and solder is very obvious on the surface of the ball.
Why use plastic balls on packages?
Use of the plastic balls can improve the solder joint reliability
As the balls do not collapse during reflow the stand-off height of the package is maintained even on via in pad designs
Balls can be assembled and soldered using existing mass placement systems
Less likely to be damaged and deformed
The balls are lighter
There are a number of reliability papers circulating and a couple of European projects have looked at using different types of ball. One used a polystyrene core with melamine resin, nickel and then gold plating. There will be a benefit to reliability of the defined stand-off height, however, the solder coverage on the surface of the ball will have an impact as will the thickness of the barrier plating layer and their rigidity.
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