Quality Check of electronic Assemblies by AXI

05 July 2013

2D x-ray: Superimposed solder joints on PCB top and bottom sides make evaluation impossible.

The trend towards space-saving opportunities in electronic circuits has led to new component types with hidden contact faces.

Combining these with double-sided placement enables compact and high-performance assemblies to be produced. This trend sets new challenges to test and inspection technologies.

Hidden solder joints (e.g. BGA components) can only be evaluated by ‘looking though’ a PCB. X-ray inspection technology is used where conventional AOI systems find their limits. 2D x-ray systems are not set to inspect double-sided equipped PCBs. Such systems‘, inspection images show solder joints of both assembly sides as su-perimposed (image 1). Consequently, a detailed process-reliable evaluation is im-possible. 2.5 D inspection technology fits better but not ideally. In this case, an an-gled PCB X-Ray, dependent on radiation angle and PCB thickness, enables separa-tion of the assembly top and bottom sides’ solder joints. If component placement on the PCB is highly compact, a simple angle X-Ray is not sufficient. Similar to 2D tech-nology, superimpositions occur, disabling the safe evaluation of the solder joint.

Digital tomosynthesis

GOEPEL electronic’s OptiCon X-Line 3D x-ray inspection system works by digital tomosynthesis. It enables constant PCB X-ray capturing from several different an-gles. Captured 2d images are converted to 3D by algorithmic reconstruction. In this manner, a separation of the PCB’s top and bottom solder joints is enabling even high density placement inspection. Similarly, superimposed and electrically connected assemblies of package-on-package (PoP) construction can now also be inspected. The 3D reconstruction layer-by-layer enables a separation of top and bottom pack-ages as well as succeeding image processing evaluation.

Table 1 provides an overview detailing the applications of different x-ray inspection methods with regard to a specific inspection task as well as utilised production tech-nology. It is evident that the inline production of double-sided PCBs needs 3D X-ray inspection.

Applications of x-ray inspection methods

The OptiCon X-Line 3D’s software and hardware concept provides all the inspection images calibrated to grey value and correct geometry. The geometry true PCB layer reconstruction allows for the utilisation of a consistent component library, resulting in a significant reduction in test program generation effort. Hence, users are able to generate a test program by means of CAD data and component entries in the library within shortest time.

PCB warpage compensation

A true to height reconstruction of PCB layers in which solder joints are situated is a prerequisite for solder joint analyses. A 3D x-ray inspection (AXI) system must detect and compensate for PCB warpage, waved surfaces or skewed components. For ex-ample, if PCB warpage is not spatially compensated, the algorithms test in the wrong solder layers, which may lead to false calls or even escape. GOEPEL utilises a pur-pose developed method, which detects and compensates PCB sag during the in-spection process. The result is a true to height solder layer reconstruction for a high-quality evaluation of solder joints. Based on PCB condition it is possible to select whether the entire assembly or each partial circuit of a value should be compensated.
For PCB warpage determination, a laser triangulation sensor can be applied. It is also possible to use software algorithm’s to compensate for PCB sag directly in the recorded images.

Image 1: 2D x-ray: Superimposed solder joints on PCB top and bottom sides make evaluation impossible.

Table 1: Applications of x-ray inspection methods

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