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In Solder Rework/Repair, Solder Training and Tools

BGA Inspection

Visual inspection of 100% of the BGA joints is not possible on these area array devices. However, there are two techniques, which if used in combination with one another, can greatly reduce the amount of defects which "pass" BGA rework inspection. With a combination of both transmissive x-ray and endoscopic inspection, very few defects of BGA rework cannot be identified.

Transmissive x-ray can be used to document several potential defects. Solder bridges are the most common defect which can be detected by this technique. Gross voids can be seen at greater power and magnification levels. In addition the concentricity, pitch, circularity and solder ball diameter can also be measures as part of the BGA inspection process. However BGA x-ray inspection has limited ability to detect opens.

A strong supplemental tool to x-ray, an endoscopic inspection tool, allows hidden solder joints or joints that are in close proximity to other nearby parts to be inspected and characterized as part of the outgoing BGA inspection process. The ability to "see" whether the reflow process delivers a defective or a target joint condition can be accomplished with this BGA inspection tool. One of the major benefits of using an endoscope is its ability to "see" underneath the ball grid array and the surface of the individual solder balls. Clues to the quality of joints including its texture, uniformity, smoothness, color and brightness and surface characteristics can be documented with this BGA inspection tool. A poor solder joint with micro cracks on its surface could be "seen" with the endoscopic inspection tool while not showing up on x-ray.

Yields for reworked BGAs have been established at greater than 95% using a paste printing process. Part of this high rework yield is the proper BGA inspection process. Supplemental SIR testing has shown that reworked components can have the same surface insulation resistance. When using the StencilQuik™ rework process SIR testing has shown an increase in surface resistance value by a factor of ten. Testing has shown that properly reworked BGAs joints exhibit the same resistance to thermal fatigue as product that has gone through initial assembly.

"Course: BGA REWORK - The Process of Inspection"

BGA Reflow Profile - Part Reflow

 "Course: BGA Rework-A Primer-Reflow"

After part placement, the applied solder must be reflowed to attach the BGA to the PCB pads. Preheating the entire PCB to between 70°C and 125°C is critical to minimizing PCB distortion during the heat cycle. The thermal profile already developed should have the following basic characteristics:

  • Peak joint temperature to minimize Pb dissolution
  • Minimum joining temperature high enough to ensure good wettability
  • All joints to be in the solder manufacturer's specified reflow range
  • Ensure dwell time above reflow as per the solder paste manufacturer's specifications

In addition care must be taken to ensure that adjacent components are not damaged during the reflow process. This care must be extended to components with low melting points such as connectors or standoffs or adjacent moisture-sensitive devices. Care should also be taken to not overheat any internal thermal grease if the BGA is capped.

After successfully attaching the BGA to the PCB, and allowing the board to be cooled down, the perimeter row of solder joints should be visually examined. The solder joint between the ball and the PCB pad should be similar in size and shape to the ball/part joint. If a water-soluble paste is used, the reworked area needs to be cleaned. Cleaning parameters should those be used in the original assembly process.

Card warping and bowing should be minimized by the BGA rework process. This can be a serious problem in the case of very thin PCBs. The bowing can be exacerbated by the proximity of other components such as pin-in-hole connectors that may anchor the PCB, thereby enhancing the localized bowing under the BGA. If the PCB is bowed or warped they might not be able to properly fit into the final assembly or opens may be formed.

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