A study finds nozzle performance is tied to diameter.

When soldering assemblies with a small number of through-hole components alongside large numbers of surface mount components, selective soldering shows advantages in assembly quality compared to traditional hand soldering or wave soldering with pallets. One of the key variables affecting joint quality is the nozzle type. The flow of Pb-free alloys and their temperature will be affected. Today, there are two types of selective nozzles: non-wettable and wettable.

When non-wettable nozzles are used, solder alloy flows from the back to front of the board, as in wave soldering. Soldering on an angle is typically required to reduce bridging defects. Novel equipment enables horizontal soldering by preventing bridges from forming. Minimum maintenance is required for non-wettable nozzles, but clearance with adjacent components becomes more critical. When wettable nozzles are used, the board can be soldered from all directions and left horizontal. The nozzles require periodic cleaning to maintain performance.

Both nozzle technologies can be employed for the assembly of Pb-free boards, but are characterized by different process windows and capabilities.

We studied the use of wettable and non-wettable nozzles with different diameters (4, 6, and 8 mm) on the assembly of Pb-free (SAC 305) boards using a selective soldering machine. Nozzle diameter is critical because it affects through-hole penetration, especially for thicker PCBs where an increased amount of latent heat is required to achieve acceptable quality and yield. Wider nozzles are able to provide this increased latent energy, but their size is limited by adjacent components. This investigation included not only nozzle parameters, but also equipment parameters such as drag speed (1 or 2 mm/s) and solder temperature (280ËšC, 300ËšC and 320ËšC).

This study used 72 16-layer, 0.093"-thick boards with copper OSP surface finish and through-hole connectors. Three types of Pb-free through-hole components were used. Four 64-pin connectors with gold finish, three DIPs with 100% tin matte finish, and 25 axial resistors with 100% tin finish were assembled per board. All component leads, with the exception of DIPs, had 1 mm lead protrusions; the DIPs had none.

A selective soldering machine with a dropjet fluxer, two IR lamp heating zones, and a select wave was used. After assembly, boards were visually inspected. Results showed that bridging was the dominant defect. With the non-wettable nozzles, less bridging was observed when the solder temperature was above 300ËšC and nozzle diameter was 4 or 8 mm. Other factors were not significant. For the wettable nozzles, diameter of more than 6 mm and drag speed of 2 mm/s were recommended.

X-ray analysis was used to inspect through-hole penetration. The board design included through-hole designs attached to different layers and different pad-to-hole ratios, which affect joint formation. All DIP and resistor connections were studied. For the pin connector only, vias connected to eight layers were inspected. The output of the inspection was 0 and 1. The value of 0 corresponded to through-hole penetration of less than 75% and 1 if it was more.

Figures 1 and 2 show plots for the mean percentage of good through-hole joints based on connection and component types for wettable and non-wettable nozzles, respectively. In both cases, the DIPs and resistors showed more instances of bad joints, especially when the vias were connected to four or more layers. For the non-wettable nozzles, the pin connector showed more instances of poor joints when larger hole diameters (>0.043") and 0.070" pads were used. Results favored the use of 4 mm and more than 300°C solder temperature. In the case of wettable nozzles, connection type had a minor effect on through-hole penetration for the pin connectors. Results favored the use of 8 mm nozzles and temps. above 300°C.

(In the figures, the first number relating to connection type corresponds to the number of layers; the second and third numbers are the pad diameters in mils, and the last two numbers correspond to the hole diameter in mils.)

Both types of nozzles are able to produce solder joints with less bridging and acceptable through-hole penetration. In the case of thick and complex boards like the one used in this study, solder temperatures of more 300°C are required, but lower operating temperatures are possible for less-complex boards in terms of board thickness, layer count and copper gauge.

The main difference in performance for the nozzle type is the nozzle diameter. Non-wettable nozzles work better with smaller diameters (0.004"), while wettable nozzles perform better with larger diameters (0.008"). It was also observed board design had a major impact in through-hole penetration, especially for DIPs and axial resistors. Most favorable designs called for fewer than two layers. The non-wettable nozzles were also more affected by board design. In the case of the pin connector, smaller hole diameters (max. 0.039") and smaller pads (0.060") were recommended.

Both types of nozzle technologies require an optimized process window. The result is acceptable solder joint penetration with minimal bridges. There are many advantages for both nozzle types, and proper selection will depend on the application.

Ursula Marquez de Tino is a process and research engineer at Vitronics Soltec, based in the Unovis SMT Lab (vitronics-soltec.com); umarquez@vsww.com.