There are several types of Ball Grid Array (BGA) inspection systems on the market today. These Tech Tips are for the optical inspection systems that use endo-scope or bore-scope technologies with a backlighting system. These systems utilize an adjustable magnification collar which allows a technician to focus on a joint a few rows into the BGA.

• Choose a scope that is applicable to your product. Be aware of the distance required between components to allow the scope head to inspect the BGA. This distance is commonly known as the “keep out” area. Some scope heads need more clearance than others, and the distance may vary depending on the manufacturer's specifications.
  
  
• The Stand Off Height, which is the distance from the circuit board to the scope head, is important. It is always better to have a wide view of the board surface for alignment and cleanliness reasons. Stand off height is also important in micro BGA and Flip Chip inspection due to the smaller solder ball size and reduced scale of the component.
  
  
• Measuring software is commonly included with most units, and should be used to ensure that the system is calibrated for accuracy. It is certified to a standard such as Six Sigma or other industry standards. To calibrate a system you will need a known value, which is sometimes supplied by the manufacturer in the form of a calibration block. If no calibration block is provided, you can use a known value or scale. When calibrating in most cases, it is important to recognize and maintain magnification so that the measurements are as accurate as possible.
  
  
• Use the systems database to bring up comparable images of BGAs and use them as references to detect any defects during inspection. Some systems will provide corrective actions in a defect library and most systems will allow you to enter your own images and corrective actions.
  
  
• When performing an inspection, you may need to measure the standoff height in several locations of the component to determine if any package or board warping has occurred. You must measure from the joint to the board interface and from the joint to the component surface.
  
  
• When inspecting joints in the center of the package, adjust the backlighting to allow for a clearer silhouette of the joint to detect opens and shorts.

Some of the important things to keep in mind while performing a visual inspection are rough, dull, or porous surfaces; surface deformities like holes, streaks, scaling and ridges; and micro-cracking, de-lamination, fractures, discoloration, micro-balling, solder splash, excess flux residue or dross inclusions. Examples of diagnosed defects including Open Solder Joints, Exposed Voids, Elongated Joints and Micro-Cracks follow.

Open Solder Joints (Figure 6-1) are caused by pad contamination, insufficient paste deposition, board and component warping, co-planarity issues, missing balls, poor fluxing, and excess or insufficient temperature conditions.

Exposed voids (Figure 6-2) in solder joints can be caused by over-temperature conditions, or by excess flux being present in the re-flow process. When inspecting a BGA, the appearance of a semi-circular silhouette can be an indication of voiding.

An elongated joint (Figure 6-3) stems from component or board warpage. Warpage can be caused by moisture in the component substrate, excess temperature conditions, co-planarity issues, insufficient paste deposition, and board movement during the re-flow state.

A micro crack (Figure 6-4) is caused by stress on the board or component after the reflow process. The stress is induced thermally by rapid cooling or mechanically by twisting the boards or components. The characteristics of a micro crack are jagged edges and intermittent operation of the package during electrical testing.

These tips are designed to assist with the BGA optical inspection process. In most cases, defects in BGAs are corrected by the removal and replacement of the component.