Introduction for PCB Assembly defects of BGA defect

Due to the BGA’s complexity, many potential defects can occur during the soldering process for a BGA. Some problems that may arise include the following:
1.Misalignment: Misalignment happens when the PCB and BGA are offset during reflow and connect at incorrect points. This is one of the most common issues that occurs during BGA assembly.

2.Inconsistent standoff height: BGAs are set on top of PCBs, and improper soldering can result in the BGA setting at a crooked angle on the surface of the PCB. This asymmetric standoff height can compromise the security of connections.

3.Missing balls: If balls are missing from the BGA when it is attached to the PCB, essential connection points may be missing from the assembly.

4.Non-wetted pads: In some cases, reflowed solder paste may not wet the pad correctly. This may result from incomplete reflow or leftover solder resist from previous steps in the production process.

5.Bridges: Bridges occur when extra solder paste is left between paste deposits. Bridges between connection points often result in shorts.

6.Partial reflow: In some cases, the reflow may not completely cover the board. This often occurs because of insufficient solder reflow, human error or mechanical malfunctions.

7.Popcorning: Popcorning occurs when balls merge together during the soldering process. This is a common problem for BGA and, like bridges, results in shorts.

8.Open circuits: Open circuits occur when solder fails to wet the PCB pad and climbs onto solder balls and onto the component surface. While an electrical test can identify this as a problem on a board, it cannot identify the cause of the issue.

9.Voiding: Voiding occurs when flow stops short of a solder connection. This is a common problem in collapsible BGA components.

These issues can all be fatal errors for the final product, so catching them with quality inspection techniques and technologies is a necessity. Fortunately, modern inspection techniques have advanced to a point where most of these defects can be identified using one or more inspection methods. However, it is also essential to avoid these defects wherever possible before assembly.

Design considerations to avoid BGA defects

While inspection techniques have advanced to a point where they can catch many defects before they proceed through the manufacturing process, it is always ideal to avoid irregularities. This requires careful planning from the design team. To maximize the chances of achieving successfully soldered joints on a BGA, design teams should consider the following in the PCB layout:

--BGA footprints: PCB designers need to make sure they have the correct footprint pattern for the BGA device they wish to use. Ensure the footprint is correct and take care to design the PCB footprint pad precisely to avoid shorts and solder issues during assembly.

--Thermal management: The PCB layout must consider thermal mass. Grouping BGAs closely together can cause a thermal imbalance between highly populated and less populated parts of the board. For example, applying enough heat to properly solder closely grouped BGAs may result in burns in less populated areas. Conversely, applying sufficient heat to avoid burning less populated areas may result in voids in the BGAs.

--Via placement: Vias or through-holes are common features of PCBs, but the placement of vias can affect the behavior of solder in the BGA assembly process. Vias adjacent to a BGA pad can result in excess solder running from the pad to the via, causing a short. To avoid this, tent any vias adjacent to a BGA pad with solder mask.

On top of these design considerations, you should also take care to develop and verify a proper reflow profile before assembly. This involves using a profiler to measure and log the temperature range at the BGA ball/pad junction when the board passes through the reflow oven. This can be done with scrap bare PCBs and BGA parts. This data can be used to ensure the reflow profile for the paste and the thermal profile of the PCB is correct, plus identify any design issues before assembly.