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By Gene Fujita, Omron Electronics
Auto manufacturers demand that the incoming electronics boards for these critical transport, safety, and entertainment systems have zero defects. Quality protocols must deliver the specified results with lowest possible investment to stay profitable. Many Tier 1 suppliers inspect boards for defects in solder paste printing, component placement, and solder joint formation, but cannot easily inspect odd forms like connectors to a wire harness. The most reliable AOI used directed light to simulate 3D views like a topographic map, and new algorithms are increasing inspection automation for odd forms.
If you bought a 2009 car, it most likely has over 50 printed circuit boards (PCBs) controlling everything from the instrument panel and audio system to engine performance and collision avoidance systems. Todd Hubing, Michelin professor of vehicular electronics at Clemson University, identifies 18 critical electronic automotive systems in his "Automotive Circuit Board and System Design for EMC" workshop example. As seen in Figure 1, electronics are found in virtually all areas of the modern car. "Automobiles are complex electronic systems," says Hubing.1
Auto manufacturers demand that the incoming electronics boards for these critical systems have zero defects. Tier 1 suppliers are responsible for implementing quality protocols that reliably deliver the specified results with the lowest investment in machinery and operating personnel to stay profitable and retain coveted contracts. Many electronics suppliers in the automotive sector now use AOI to inspect boards for defects in solder paste printing, component placement, and solder joint formation.
For optimum in-line inspection of SMT boards, some Tier 1 suppliers use a combination of three AOI systems: one placed after solder paste print, one after component placement; and one after reflow soldering. Newer chip mounting machines provide built-in placement verification systems, eliminating a separate AOI system and step. Most suppliers inspect after reflow soldering. Those concerned with reducing rework delays and costs have added AOI for solder paste, also called solder paste inspection (SPI), to verify proper shape and location of the deposition.
The most reliable technique for inspection uses directed light to simulate 3D inspection views for image analysis. One system uses a color highlight technology that color codes solder height, as on a topographic map. This robust approach easily adapts to the change from eutectic to lead-free solders. Changes in image-analysis algorithms have enabled AOI systems with color highlight technology to focus on the ratios of color density to maintain accurate inspection and eliminate white-out reflections that could obscure defects. The development of high-speed, true-color (red/green/blue RGB) analysis has maintained the maximum image information to accommodate in-line production speeds without conversion to grayscale.
The Final Frontier Odd FormsAs Tier 1 suppliers seek ways to apply objective standards to all aspects of their production, board inspection after soldering of large connectors for wire harnesses has defied automation. The finished SMT boards go through a second process of connector placement and partial wave soldering. Adding the large connector required for the wire harness is either manual or semi-automatic. The boards then go through a partial wave solder process that can cause solder splashes and splatter onto SMT components, potentially creating shorts. Because boards are often hand-fed into the wave solder equipment, contaminants and other debris can be deposited on the assembly.
The post-solder inspection continues to be a manual process with difficult-to-maintain standards of quality. In factories where an SMT line produces several thousand boards a day, with multiple lines running, this final board inspection creates a bottleneck. Developments in post-solder inspection system technology have enabled semi-automatic inspection with a consistent core of characteristics. The re-checking process identifies misplaced solder and rubbish between leads and parts. A large automotive supplier in Japan qualified the inspection algorithm for this process over five years of use. What is normally single-digit percentage rejects were reduced to a fraction of a percent with minimal escapes.
ConclusionWith human inspection, there are no specific standards for quality. The automated inspection process can set a threshold for detecting defects and maintain consistent results through a whole shift. This reduces the labor cost of inspection, reduces human errors in inspections, and shortens tact time for faster inspections. Inspections can be handled in-line as well as semi-automatically with escape rates near zero. Inspection systems must improve to bring automation and consistency to the varied assembly processes, such as connector attach, and to stay at in-line speeds.
REFERENCES1. Hubing, Todd, "Automotive Circuit Board and System Design for EMC," http://www.cvel.clemson.edu/workshop/pdf/AutoEMC-Workshop-Hubing.pdf.