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88 SMT Magazine • April 2016 With laser sensors the warpage can be mea- sured and corrected with the robot z-position. Most of the assemblies have holes and slot- ted holes at the front and rear ends of the board; these can be used for positioning. The pictures show how pins of a multi plate fit into the holes and guarantee the position of the assembly in the hot solder pot. Temperature is another parameter that af- fects soldering. Temperature is continuously monitored and logged for management infor- mation and, traceability. The solder temperature depends on ther- mal heat of the assembly, hole filling issues, pin- to- hole ratio, and solder alloy. Solder temperatures in selective soldering processes are relatively high and the volume of solder is small. This makes copper dissolution and other contaminants like nickel more critical concerns. To ensure that the composition of the solder doesn't run outside of specifications, a sample of the solder should be sent for inspection ev- ery month. Once the trends are known, the fre- quency of sampling can be reduced [4] . Solder temperature and contact time are the most relevant factors impacting hole filling of a solder joint. Contact time is another factor that can't be set as a machine parameter, but it is a derivative of several parameters. The solder level, pump speed, board warpage, wave height measurement and robot speed all have an im- pact on contact time. In a dip soldering process, the solder is pumped to the solder side of the board and the pump speed is lowered before the board moves away. Contact time is programmed, but it de- pends on acceleration and deceleration of the solder pump. During a wave height measure- ment, a needle defines the height of the solder and may correct the pump speed if required. This results in a consistent contact time. The solder level in the pot must be con- trolled. One bar of solder (1 kg) increases the solder level with 1.4 mm. How much this in- crease impacts wave height depends on the number and sizes of the nozzles. Thus, a wave height measurement with a needle is an ad- equate method for all different nozzle plate di- mensions and designs. The contact time itself, the final parameter that counts, is measured in a machine capabil- consisTEnT conTrol ovEr ThE sElEcTivE soldErinG ProducTion ProcEss Figure 13: Camera with software to measure fiducial position. Figure 14: Pins on the multi wave plate serve to flatten the board and the slotted pin to position the assembly. Table 4: Solder position recommended controls.