Solder Sphere Production Method Ensures Package Integrity Print E-mail
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Written by Dr. Renzhe Zhao   
Monday, 31 December 2007 19:00
Materials World

Array package production, in particular BGA and CSP package manufacture, has increased nearly 25% compounded annually over the past decade and is forecast to maintain that rate. Simultaneously, devices are packing in more functionality, yielding higher I/O counts and finer pitches. One critical component of the package success is, obviously, the integrity of the bottom-side solder spheres used to connect these devices to the carrier. Surprisingly, no worldwide standard defines the quality of solder spheres, and packaging firms are left to rely on the sphere manufacturer’s production and quality methods and their own analyses.

A novel sphere production method, however, is delivering spheres with a level of quality, repeatability and control not achievable through traditional manufacturing methods. When evaluating solder sphere quality, one must assess three main factors: the sphere manufacturing process, the finished spheres’ oxidation levels, and the spheres’ geometric shape. All these conditions impact the yield, performance and reliability of the end-product – the BGA or CSP.

Conventional sphere production methods incorporate a mechanical process whereby small metal particles are cut or punched out from fine wire or metal sheets. The particles are dropped into a hot oil bath where they are melted to form small round drops. As the oil cools, the droplets solidify into spheres. This procedure has intrinsic limitations that result in coarse dimensional tolerances because each mechanical operation adds a certain amount of deviation to the size and uniformity of the particles that, together, produce an unacceptable cumulative effect.

The other factor affecting sphere performance is what is commonly referred to as the “blackening effect” in short: oxidation. It is well known that spheres turn dark after they collide against each other and the container wall during handling, storage and shipment. The oxidation, if severe and not amended during reflow due to insufficient flux or too thick an oxide layer, can be detrimental. This condition can cause an inadequate solder bond between the sphere and its corresponding substrate solder pad. Minimal oxidation is obviously the most ideal condition.

Last, but certainly not least, is sphere geometry – diameter and roundness. Most sphere suppliers determine the diameter of their spheres by measuring in two directions, x and y. This is not optimal, as it is easy to miss the largest of the smallest diameter of the sphere. Roundness is also a measurement that should be taken into account and is employed by very few sphere suppliers. The sphere, for maximum performance, should be nearly perfectly round. Sphere geometry is important for many reasons. First, today’s sphere deposition equipment is very precise and any odd-shaped spheres may cause the equipment to jam, thus diminishing throughput. Second, if spheres used on the same BGA have different diameters and the differential is quite large, coplanarity issues can be the result.

To address such issues, new technology incorporates a unique sphere manufacturing method involving a mechanical jetting procedure and innovative sorting method. To combat oxidation, a process was developed to reduce the amount of surface oxygen and, therefore, minimize the “blackening effect” and increase sphere shelf life.

Diameter and roundness are also addressed with some unique perspectives. Our company believes that two measurements to determine sphere diameter are simply not enough. In our view, at least 10 measurements – sometimes more – should be taken and the diameter of the sphere is the arrived upon based on average value of the multiple measurements. In addition, a roundness value (R) is determined by dividing the difference between the largest diameter measurement and the smallest diameter measurement by the sphere diameter. As a rule, a sphere’s roundness factor will be considered good when its R value (based on the above formula) is less than 0.033.

Packaging specialists should question their sphere suppliers on the safeguards and procedures being used to protect against oxidation. Equally as important is the consistency of the diameter and roundness of the spheres – especially as we move to increasingly miniaturized packages and finer pitches: sphere quality, reliability, consistency and performance must be guaranteed to ensure high yield packages. In this environment of smaller, better, more cost-effective, there’s little room for error. Make sure your solder spheres (and those on the finished devices you are procuring) are manufactured with the latest technology or you might be reballing those packages!

Renzhe Zhao, Ph.D., is technical manager of applications engineering at Henkel’s Electronics Group (henkel.com); This e-mail address is being protected from spambots. You need JavaScript enabled to view it .

 

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