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Cracked Solder Joints

While out shopping the other day, I noticed a fairly new LED type traffic light, where several of the green LEDS were not working.  I think I know the reason for this.  Over the last several years, traffic lights have been shifting from classic incandescent lamps to LEDs. 
The objective was to make the light assembly weigh less and last much longer than the incandescent lamp systems.  This translated to fewer repair costs over a 10 to 20 year period and a lower energy bill.  However, many of the cheaper lights are made in China, where quality control is almost nonexistent.  I would bet that the circuit board on these defective LED lights contain only one solder side. Single sided boards are not good for outdoor applications where temperature swings are extreme.  The temperatures can put too much stress on the solder joint, leading to cracks as shown below.  A double sided board is much stronger.
Traffic Light with Bad LEDs

As illustrated below, in a double sided circuit board, the solder forms an eyelet around the component lead.  This prevents the lead from pulling through and dramatically increases the solder to component lead contact area.  The result is a very strong solder joint which can tolerate temperature swings. 

Double Sided Circuit Board
 
Crack in Component Solder Joint Crack in Component Solder Joint

July 2010     Issue 11

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