What kind of defects can you anticipate if BGA assembly isn’t properly performed? Defects are due to a number of reasons. Included are under-etching, and ineffective thermal profiles.
A thermal profile may not be at a sufficiently high enough temperature. If a lead-free BGA is being processed based on a leaded thermal profile, definite issues will arise. A leaded or eutectic profile is about 25 to 30ºC lower in temperature for melting points than that of a lead-free profile. So, in this instance, an open ball problem can result from using the wrong thermal profile.
The opposite is also true when processing a leaded board based on a lead-free temperature profile, which is considerably a higher temperature. The result can be the shorting of pins. Also, if the board is under-etched, the ball size is bigger, which in turn can cause a short. If the board shows the slightest bit of warpage, then again it can cause a shorting of pins or balls.
Therefore, advanced tools, methodologies, and procedures are critical during assembly to catch and minimize defects and associated problematic areas. Your EMS provider should have extensive experience in using AOI versus X-ray inspection and a proven track record for effectively using these and other tools.
Silver-filled pad related defects pose one particular area of assembly concern. Here, the OEM should take time to understand BGA inspection to determine how a board behaves in terms of silver-filled pad problems. In this scenario, there is a BGA pad and within that pad a via hole is made and then filled with silver.
At times, this is not properly performed since it is a process within a process. What occurs is the via is not properly filled with silver; a known flat surface is created; and it causes joint integrity issues. The via can drop down, leaving a pad with a hole in between.
So, instead of the solder sticking on the board’s surface, it will flow through the open hole to the opposite side of the board to create an open ball. This is a ball which is supposed to melt and collapse on a pad. However, now, solder has dripped through to the other side, hence either a marginal or no connection is made.
A void is another common defect. This is where the BGA is not 100 percent collapsed and a gap exists. The gap could result from multiple and different factors and reasons.
One is the lack of localized heating on the BGA, itself. Sometimes, a thermal profile is not optimal, which can create a void. According to the IPC, different classes have different acceptable limits. IPC Class II allows as much as a 25 percent void as being acceptable.
This means that if ball size is 100 percent and there’s a gap of up to 25 percent of that ball size, then according to IPC Class II, it is an acceptable BGA joint. Under IPC Class III, a 12 to 15 percent void is acceptable, and under Class I, 40 percent.
A void means there’s no connection to a particular solder joint region on a BGA ball. However, if the void is within acceptable limits as stated above, then it is good to go. But if not, then an issue arises.
Advanced X-ray and supporting software can calculate such a void and determine if it is acceptable or not. There are also third party service bureaus that do this work and provide well-produced calculations for every void.
2D X-ray is a key inspection tool to help uncover voids and other defects. It effectively covers about 80% to 90% of PCB applications, but in about 10 to 20 percent of cases, 3D X-ray is needed and comes with a cost premium. Moreover, a highly skilled technician is required to operate 3D X-ray.
On the other hand, an advanced 2D X-ray with the right software can render 3D images to achieve virtually the same capability as that of a more complex and expensive 3D X-ray. This way, the OEM eliminates the extra cost associated with 3D.
AOI is another vital inspection tool, however it has its limitations since it cannot view the underside of a BGA. It can look at the polarity and the markings on top of a BGA to verify part numbers. Therefore, X-ray is the more efficient of these two inspection tools.