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In Solder Rework/Repair, Solder Training and Tools

Visual Inspection

A strong supplemental tool to XRAY, a endoscopic inspection tool, allows hidden solder joints or joints that are in close proximity to other nearby parts to be inspected and characterized. BEST has the capability to inspect and troubleshoot process defects through the use of this tool.

Inspection SystemInspection System
Detection of Joints

While there are several destructive methods (SEM or pull strength testing) for detecting the presence of defective solder joints underneath sections of grid array packages, the endoscopic inspection tool serves a unique and useful purpose. A solder joint can prematurely fail due to the shear stresses caused by the Coefficient of Thermal Expansion mismatch between the ceramic material and the substrate is substantial. The ability to "see" whether the reflow process delivers a defective or a target joint condition (see figures below) can be accomplished with this inspection tool. While this joint would pass a test for shorts and opens during electrical test, life cycle tests would reveal a different story.

 

Solder ball
Examining the Surface of the Joint

One of the major benefits of using such an inspection tool is its ability to "see" underneath the grid array and the surface of the individual solder balls. Clues to the quality of this joint include its texture, uniformity, smoothness, color and brightness and surface characteristics. A poor solder joint with micro cracks on its surface could be "seen" with the endscopic inspection tool but would not show up on XRAY. Flux residue spread between the balls underneath the array can lead to bridging of conductive particles and a shorting between pads. This can be discovered with the use of this inspection device.

Lead free solder lift offLead free solder lift off
QC-Is it a Good Solder Joint?

The endoscopic inspection system with its accompanying image capture software can be used to gauge a go/no-go decision in terms of the quality of the solder joint. By using window overlays of good/bad reference pictures the joints can be inspected as part of the quality control process. Critical ball dimensions can be measured in terms of their stand off heights, wetting angles, pt-pt distances in order to confirm the "quality". These images can be captured and trended for capability studies or failure analysis. These measurements also add to the objectivity of the soldering process. Over time a large database containing the various critical process parameters involving soldering (flux, paste temperature profile, etc) can be stored and subsequent troubleshooting becomes faster. Component and equipment suppliers as well as end customers can be shown these images in order to effectively communicate the status of the process development.

X-RAY Inspection

XRAY imaging is a widely utilized analytical tool for the SMT PCB manufacturing process. BEST will inspect your area array packages such as CGAs, BGAs, CSPs and LGAs via a transmissive X-ray inspection to ensure the rework meets the highest quality levels. This is a critical step in the quality control of the BGA rework process as high resolution, high magnification and high contrast X-ray images will confirm that the boards have been reworked per the industrys’ acceptance criteria.

XRAY imaging, given that well-trained technicians are operating the equipment and interpreting the results, can detect a multitude of processing problems including:
  • SHORTS are found using X-Ray inspection. Trained operators will make sure to note that on some double sided boards a component from the backside may give the appearance of a solder bridge. By rotating the board along the X and Y axis the true image can be seen and properly inspected.
  • POTATO CHIPPING occurs when a component's outside edge lifts up from a pad. This will cause the center joints to appear “flattened” and non-uniform as a result of overheating the component. This defect can be found by rotating the board on its X and Y axis allowing the x-ray technician to see the hourglass appearance of the outside edge and corner solder joints
  • VOIDS are the result of improper reflow profiles which exhibit entrapped flux gasses. The best way to view voids is to use a lower voltage and amperage, while the board lays flat along the X and Y axis. The technician will either manually or with the aid of analyzing software determine the percentage of void and match that against the IPC inspection criteria.
  • POPCORNING is the breaking open or de-lidding of the package top from within the package due to moisture. The eruption may break the die from the substrate, it may pull wire bonds from pads or delaminate the package substrate itself. The x-ray technician can spot this defect when extreme cases of popcorning occur.
Some of the more common ones are illustrated further below:

SOLDERING TIPS

soldering-img

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