Avoiding embedded PCB design defects
A number of different solder defects and associated failures in embedded designs are becoming more prevalent 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 0.25 millimeter (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 center of one BGA ball to the center 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:
- Cracked ball
- 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- Out-of-date assembly floor personnel training, and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.
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 3 shows the bridging defect that occurs when the thermal profile is too hot.
‘Tombstoning’ (Figure 4) is created due to the solder’s surface tension during reflow. As a result, one end of the component is detached from a PCB’s copper pad and vertically lifts up, resembling a tombstone.
A micro hairline crack on a BGA ball (Figure 5) is due to solder-joint fatigue damage caused by thermo-mechanical and shock stresses.
Figure 5: Micro hairline crack on BGA ball is due to solder-joint fatigue damage caused by thermo-mechanical and shock stresses.
Figure 6 shows the ‘head-on-pillow’ defect. In this case, the deposited solder paste only wets the pad. However, it doesn’t fully wet the ball. As a result, the solder joint has a sufficient enough connection for electrical integrity, but doesn’t have enough mechanical strength, thus making it a candidate for a failure in the field.
Figure 6: ‘Head-on-pillow’ defect is caused when deposited solder paste only wets the pad but doesn’t fully wet the ball.