Controlling the solder angle can reduce bridging.

In a selective process, the assembly is dragged through a small wave former. The solder does not flow as it would in a wave soldering process, so the solder temperature must be higher to achieve proper through-hole penetration, but not so high that it might burn off the flux and damage the assemblies.

SAC and SnCuNi are the most popular Pb-free alloys used in selective soldering. Compared to SnPb, the different surface tension and density of these alloys result in different flow behavior through the nozzles. This flow should be in control; otherwise it results in bridging or may affect surrounding surface mount components. These Pb-free alloys tend to flow in the same direction the board is moving, resulting in bridging. To reduce this defect, a small experiment was run. A nonwettable, 6 mm diameter nozzle was used (Table 1).

The board was FR-4, 16 x 10 cm and 1.6 mm thick, with ENIG surface finish. The components had 20 pins, a lead length of 1.5 mm, and a pitch of 2.54 mm. The flux was a commercial no-clean alcohol-based flux and the alloy was SnCuNi. Nitrogen supplied to the soldering area was set at 50 l/min.

An L9 Taguchi experiment was run with four replications. A total of 36 boards were assembled. Pins that were assembled correctly were counted. In every run, the maximum score of 80 indicated there were no defects (Table 2). Runs 2 and 5 showed no bridging. Nevertheless, it was recognized that in run 2, the solder was flowing incorrectly. The influence of single factors on bridging was, in order, solder angle (35%), solder temperature (26%), nozzle-PCB distance (24%) and experimental error (15%).

Data analysis shows the best way to avoid bridging is to solder with the following settings (Figure 1):

• Solder temperature (A2): 280°C
• Speed of the robot (B1): 3mm/s
• Distance (C2): 4 mm
• Solder angle (D1): 10°

As observed in the experiment, the varying surface tension and density of SnCuNi make selective soldering very challenging. To maintain control of the solder, wave nozzle technology is an important parameter to be reconsidered for each application. As in all soldering processes, flux activity at the higher solder temperature and nitrogen use may improve solder formation as these parameters reduce oxidation.
Because small nozzles are used, much heat must be transferred through a small amount of solder into the board, connectors and pads to enable soldering. Therefore, the flux must support temperatures of at least 280°C. In this experiment, all connectors showed good through-hole penetration when soldered at 280°C.
In conclusion, small nozzles may experience problems with heat transfer; therefore, nozzle diameter is a critical parameter and may affect board layout. The higher solder temperatures used in selective soldering require stronger fluxes and resistant board material.

Denis Barbini, Ph.D, is advanced technologies manager at Vitronics Soltec (vitronics-soltec.com); dbarbini@vsww.com.