Page 70


www.us-tech.com


September, 2017


X-ray Uncovers the Best and Worst of BGA Assembly


By Dr. Bill Cardoso, President, Creative Electron, Inc. B


GA technology is touted as the ideal interconnect solution because it uses solder balls as


technologies. The solder ball can be placed as near as possible to the pad of the die, thus minimizing the total


Figure 1: This image shows most defects that can be encountered in a BGA assembly.


leads to provide a reasonable level of self-alignment. That means that there is some tolerance in the placement accuracy, which is a direct function of the size and density of the BGA. The BGA package also has con- siderable electrical and thermal advantages over other packaging


pad-to-pad impedance. Also, the presence of many solder balls, which are excellent thermal conductors, under the entire package greatly reduces the thermal impedance of the BGA.


BGA Assembly and Rework


The use of BGAs usually leads to high manufacturing yields, so if a board needs rework it is likely that the issue is related to either materi- als or processes. The first thing to check is the bare board quality. Ensure that the PCB has solderable pads and that those pads have a sol- der mask that is not encroaching excessively onto them. The next step is to make sure that the solder paste printing is done correctly. Avoid solder skips at all costs, and if needed, include a 3D or 2D sol- der paste inspection step until this process is under control. If a high-quality bare board paired with a smooth sol- der paste application process is still having issues, it is time to look at the reflow process. The fact that BGAs are great at con- ducting heat through solder balls is both good and bad: good for the thermal management of the electronic components inside the BGA package and bad for assembling it. The fully- assembled board must be care- fully profiled to minimize the tem- perature gradients between the mid- dle and outer regions of the BGA. There are several methods that rework experts suggest for this. These methods include using ther- mocouples around the BGA to map


the temperature profile of the pack- age. This way the rework machine knows that if it measures 240°C (464°F) at the top nozzle (for SnPb), the BGA solder ball is at 220°C (428°F). This 20°C (36°F) delta is not atypical.


BGA rework is similar to assembly, though it varies slightly depending on the final goal, whether it is to save the board, to save the


Figure 2: Example of a good BGA assembly.


BGA, or to save both. However, some of the key steps in these processes are the same.


Start by baking the board prior to removing the BGA. Baking the board will remove the moisture from


Continued page 72


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