September, 2019


www.us- tech.com


Fast and Reliable: X-Ray Inspection Detects and Measures Voids


Continued from page 61


their bottom sides, state that the acceptance crite- ria for these components must be determined by an agreement between the user and manufacturer. For the heat transition layer, these criteria


depend on process and design, so the component manufacturer’s application notes, solder coverage, pores, solder height, etc., must be taken into account. The formation of pores and bubbles in the thermal layer cannot be entirely prevented when soldering these types of components. Voids can prevent optimal dissipation of heat


from chips and semiconductor component barrier layers. The life span (operating hours), wavelength (color) and even the light yield (brightness) of the LEDs are strongly dependent upon the barrier layer temperature.


Strict Requirements Especially in the automotive


industry, the manufacturing and quality control of PCBs assembled with LEDs are often sourced to EMS providers. They must meet the strict requirements of their customers and, at the same time, be as efficient as possible to stay competitive. “In this case, it is the brand manufacturers who specify exactly what percentage of voids is acceptable in solder joints. The same applies, for example, to the quantity and size of unwanted solder balls, where in the case of LEDs, inline X-ray can also be used to reli- ably comply with threshold values,” says Andreas Gladis, sales engineer, Viscom. Reflow ovens with vacuum chambers and other means can help prevent voids. Those that do form can be very


clearly identified with cutting-edge 3D X-ray, in both BGAs and LEDs. While a flexible, manual system is adequate for random inspections, 100 percent fully automatic inspection is performed by a system in the produc- tion line. This way, voids can be com- pletely recorded and analyzed, even while hidden from the human eye.


Parallel PCB Handling Inline X-ray systems must be


able to reliably detect defects that are invisible. These systems undergo con- stant development to keep pace with the rising demands for speed in elec- tronics manufacturing. Today, inline X-ray inspection can achieve han- dling times as low as four seconds. This is possible because up to three PCBs are handled in the 3D AXI sys- tem at the same time. Faster 2 and 2.5D inspections


can be combined with 3D inspections as needed. Complex overlapping is particularly common for PCBs popu- lated on both sides. Here, a 3D recon- struction can produce clear slice images to provide sharp visibility of all significant defects — even with component shadowing and multilayer boards. While 2D X-ray images include


visible details from the back side of the board, 3D slice images eliminate interfering structures. This greatly simplifies the creation of inspection programs and effectively helps to pre- vent false calls. Depending on the software used, some 3D AXI systems today can even apply planar CT to generate partial or complete solder joint volumes that can then be tilted, rotated and zoomed onscreen.


X-Ray Complements AOI Nonetheless, the importance of


3D AOI in inspection is by no means See at SMTAI, Booth 816 and IMAPS, Booth 313


Inline X-ray inspection with fast PCB handling.


Page 67


overshadowed. Especially in automotive applica- tions, there are very exacting requirements for the positioning and alignment of LEDs. Defects in ever smaller LED components, such as very small non- coplanarities and misalignments, must be detected. “Today, Viscom inspection systems make it


possible to precisely determine the position of the light-emitting surface with reference to the holes for mounting to the vehicle and at the same time by taking into account the fiducial marks. For this, the PCB coordinate system is carried over to the coordinate system of the mounting holes,” says Gladis. To support the absolute precision and


repeatability of 3D AOI, a height gauge — meas- ured with high-precision instruments and certified by a specialized laboratory — can be used as a tar-


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