Solder ball evaluation of BGA package

Array packages such as BGA and CSP have grown at a CAGR of nearly 25% over the past decade and are expected to continue to grow at this rate. At the same time, device packages are becoming more functional, with higher I/O counts and finer pitches. It is clear that the key factor in the success of packaging is the perfection of the solder balls used to connect the device to the carrier's base plate. However, surprisingly, there is no global standard for solder ball quality, and packaging companies can only rely on the solder ball manufacturers' own analysis to evaluate quality.

The new solder ball production method can provide a certain quality level of solder balls, whose repeatability and controllability cannot be obtained by traditional production methods. There are three main criteria for evaluating solder ball quality: the production process of the solder ball, the degree of oxidation of the finished solder ball and the geometry of the solder ball. All of these conditions will affect the yield, performance and reliability of the BGA or CSP terminal product.

New solder ball production method provides more repeatable and controllable solder balls

Traditional solder ball production involves a mechanical process that cuts thin wires or punches metal layers to create small metal particles. The particles fall into a pool of hot oil and melt into small round solder droplets. When the oil cools, the solder droplets solidify into a ball shape. This process has inherent limitations because each mechanical operation adds a certain amount of deviation in size and consistency to the particles, which has an unacceptable cumulative effect, resulting in coarse size deviations.

Another factor that affects solder ball performance is the well-known defect effect. Simply put, it is oxidation. It is well known that solder balls will darken due to collisions with each other and the walls of the container during handling, storage and transportation. Oxidation can be very detrimental to production if it is severe and cannot be improved during reflow due to insufficient flow or too thick an oxide layer. These will cause inadequate bonding between the solder ball and the substrate pad to which it is connected. Obviously, reducing oxidation will be very important to the process.

Last but not least, the solder ball geometry, diameter and roundness. Most solder ball suppliers measure the diameter of their solder balls in both the x and y dimensions. This is not optimal, as it is easy to miss the largest and smallest solder ball diameters. Roundness is also a factor that needs to be considered, but only a few solder ball suppliers take it into account.

The best performing solder balls should be nearly round. Ball geometry is important. First, today's solder ball deposition equipment is very precise, and any oddly shaped solder ball may cause the equipment to malfunction, greatly affecting yield. Second, if solder balls of different diameters are used on the same BGA and the difference is large, it will cause coplanarity issues.

As mentioned previously, we have developed a solder ball production method that eliminates almost all of the problems, producing very round solder balls with consistent, repeatable diameters and extremely thin oxide layers. This process, patented by Henkel, uses a patented mechanical jetting process and innovative sorting methods to provide higher quality control, very low impurities (oxidation) and excellent coplanarity. To address the common problem of oxidation, we developed the Accurus process to reduce the amount of oxygen on the surface, thereby minimizing defect effects and increasing the life of the solder ball board.

Different diameters of solder balls will cause coplanarity issues

The diameter and roundness issues can also be solved using some unique methods. We believe that the two methods of measuring solder ball diameter are not simple enough. For Multicore Accurus solder balls, at least 10 measurements are required, and sometimes even more. The final ball diameter is the average of multiple measurements. In addition, the roundness value (R) is determined by averaging the maximum and minimum values ​​obtained by measuring the ball diameter. Generally, when the R value is less than 0.033, the ball roundness factor can be considered good.