Surface-mounted device (SMD) technology became the soldering standard in the 1980s. Printed-circuit board assemblies were produced by printing a paste-solution of wet solder onto an unpopulated board, using a matching stencil. Next, automated placement of the components using robotic pick-and-place machines accurately set them into the wet-solder paste. Last, the populated boards passed on a conveyor through an oven to reflow the solder (to melt the solder from paste into viscous liquid). As the assembly emerges from the oven, the liquid solder cools, and the SMDs are perfectly fused into their proper location.
While complicated, SMD process technology was mastered by many OEMs, and proved to be an effective and profitable method of solder-assembly for nearly 20 years. Then, the RoHS initiative hit the manufacturing sector very hard. Known as the Restriction of Hazardous Substances, RoHS was adopted in 2003 by the European Union, and was enforced to become law in every manufacturing country by 2006.
RoHS identified six bad substances, with lead (Pb) being named as public enemy number one. Lead was the main element in solder and has a relatively low melting point. Removing it from solder was not trivial, and would require higher temperatures in every refined process in electronics manufacturing. Suddenly, all of the proprietary I.P. and know-how was at risk, and the big OEMs were scrambling with lead-free experiments to stay ahead of the competition.
This phenomenon had a monumental impact on connector companies like Molex, Berg, and Amp, who faced a redesign of virtually every connector in their portfolios, to eliminate both beryllium from the metal leads (also banned by RoHS) and to increase the plastic connector body’s thermal tolerance for new, higher temperatures.
Of course, such sweeping changes take time, and companies still had to produce shipping products simultaneously to remain in business, concurrent with experimentation of new RoHS process methods. This overlap of two major requirements is the number one reason why so many products started experiencing cold solder joints in the field in the 10 years following 2000.
Higher temperature RoHS connectors weren’t readily available from component manufacturers, plus, major OEMS still had lots of pre-RoHS connectors in stock, which couldn’t stand the higher temperature. Couple this with the efforts of industrial engineers to redefine solder thermal profiles, and you’ll find new high-temp RoHS solder being used with older low-temp connector bodies -- and the connectors would melt.
The manufacturing quota still had to be met, so the easy way out was to reduce the heat just enough so that the connectors wouldn’t melt. The result was RoHS solder not making the full phase-change from paste to liquid (reflow) before its cool-down. Subsequently, strong joints didn’t occur. The solder would contact the electrical pads, but fall short of actual reflow, where the meniscus of the molten solder ball would break, then wick into a fully bonded attachment.
This occurrence of cold-solder-joints is clearly visible when examined under a microscope. Solder joints appear as rough, grey, semi-flattened spheres, instead of the bright shiny filleted bonds that they should be. Because a cold joint still makes contact, initial factory functional test would always result in a pass. But the slightest jarring of the product in the field would cause a breakaway of the cold ball from the pad, resulting in an open circuit.
This issue was more widespread than industry leaders would like to think, and literally thousands of products suffering this anomaly were produced and launched, including the HP printer that was baked back to life. The printer was thought to have failed, but its main board was removed and placed in an oven at 350 degrees for eight minutes. The solder successfully reflowed, and everything worked normally again. If you understand the events leading to this, it all makes perfect sense.