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Preventing embedded PCB design defects

Posted: 28 Jan 2015 ?? ?Print Version ?Bookmark and Share

Keywords:PCB? ball-grid array? BGA? QFN? PCB?

A number of different solder defects and associated failures in embedded designs are becoming increasingly common due to smaller printed circuit boards (PCBs) as well as shrinking ball size and pitch of ball-grid array (BGA), chip-scale (CSP), and quad-flat no-lead (QFN) packaging.

For example, today's highly advanced BGA packages are fitted with tiny balls ranging from 0.15 to 0.25mm (mm) in size, according to Institute of Printed Circuits (IPC) literature. BGA ball pitch, on the other hand, ranges from a standard 0.8 to a more advanced 0.25 mm pitch. Pitch is defined as the spacing between the centre of one BGA ball to the centre of the next one. As a result, there's little area left for the soldering process.

If you haven't already, it's a good idea to add the following terms to your embedded design vocabulary and take them under consideration for your next designs:
???Shorts
???Opens
???Bridging
???Tombstoning
???Cracked ball
???Head-on-pillow
???BGA intermittent connections

The following conditions increase the probability of incurring greater numbers of these defects:
???Poor solder paste deposition
???Poor stencil quality
???Less than perfect thermal profile
???Inadequate assembly and inspection systems,
???Out-of-date assembly floor personnel training, and
???Lack of collaboration between embedded designer and assembly/manufacturing engineering.

Plus, printed circuit boards (PCBs) are getting considerably smaller and tend to be loaded with greater numbers of smaller device packaging like micro ball-grid arrays (BGAs) and chip-scale packaging (CSP), as well as smaller passives like 01005, which can barely be seen by the naked eye. That means accurate solder deposition is facing new challenges.

Figure 1: Shorts defined as defects occur when placing too much solder paste on a BGA ball.

Let's start with shorts defined as defects that occur when placing too much solder paste on a BGA ball, as shown in figure 1. Conversely, if enough paste isn't applied, a cold solder joint results. An 'open', also known on the assembly floor as a non-collapsed ball issue (figure 2), results from insufficient heat applied during the reflow process.

Figure 2: 'Opens' or non-collapsed balls result from insufficient heat applied during reflow process.

Figure 3 shows the bridging defect that occurs when the thermal profile is too hot.

Figure 3: Bridging defect occurs when the thermal profile is too hot.


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