W – Wafer
While the microelectronics industry is working hard to develop low-cost LED packaging methods—such as with thermally and electrically conductive epoxies—eutectic media still dominates this lighting market. More precisely, hard eutectic solders. The coolest of all hard solders is good old 80/20 gold-tin (AuSn). Many LED manufacturers routinely back-side metalize their wafers prior to dicing.
Wafer Testing
For any single set of wafers that goes through a production run, bad die bear the same cost as good die because bad die also have to be processed all the way through the LED manufacturing steps. Wafer inspectors are capable of inspecting all patterned wafers at various process steps such as lithography, etch, and metallization.
After wafers go through FEOL (front end of line), they go on to BEOL (back end of line) where die singulation, testing, and sorting take place before final assembly. Pre-dicing and post-dicing singulation inspection is essential to enable process control and improve the yield. Post-singulation should look for defects to ensure that the singulation tooling does not damage the wafers.
X – X-axis
Precision eutectic component attach includes pick-and-place of the diode, in-situ reflow of pre-form or pre-tinned devices with programmable x, y, or z-axis agitation, and programmable pulse heating or steady-state temperature.
X-ray
To assess the mechanical construction in an LED device, an evaluation should include techniques such as x-ray photographs of LED soldering. Measuring actual thermal performance to validate thermal design assumptions is necessary to ensure quality and reliability of SSL products.
The quality of the mechanical interface between the LEDs and the PCB, which is a major influence on the SSL system thermal performance, is evaluated using x-ray photography. The x-ray analysis of a PCB is useful to verify the quality of the soldering process and can determine whether there is voiding or excessive solder present. For the x-ray images, the camera head positioned above the LED dome with the focal plane at the solder pad boundary provides a clear indication of a quality solder interface. A large amount of solder voiding and excessive solder particles between the pads of the PCB is one example of a poorly soldered LED.
Sources:
LEDs Magazine, “Improve LED manufacturing via in-line monitoring and SPC”, http://www.ledsmagazine.com/articles/print/volume-10/issue-7/features/improve-led-manufacturing-via-in-line-monitoring-and-spc-magazine.html.
LEDs Magazine, “LED system evaluation yields quality analysis”, http://www.ledsmagazine.com/articles/print/volume-10/issue-9/features/led-system-evaluation-yields-quality-analysis-magazine.html#.UjCz_pSeiOE.twitter.
Photo Credit: http://img.optics.org/objects/news/1/5/21/osramwaferblue.jpg.
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Janine Hueners
Marketing Specialist
Palomar Technologies, Inc.