Wave Solder Defects

Becanse of the extra operations and exposure of components on the secondary side to molten solder, this type of assembly is prone to many assembly defects. It is important to note that soldering fine-pitch parts on the secondary side using wave soldering results in excessive solder bridging and should be avoided. In addition, active components, such as ICs, may be damaged by exposure to excessive heating. 

Back-side actives have a high defect level when the wave solder process is used. 

Keegan, J., Lowell, N. C., and Saxeena,N., Solder Joint Defect Analysis for Inert Gas Wave Soldering, Proceedings ofNEPCON West, 1992, pp. 672 90. 

An adjunct to the surface-mount process, this method, sometimes referred to as intrusive reflow, allows the soldering of some through-hole (solder-tail) parts into plated-through holes on the circuit board during SMT oven reflow. This process can eliminate or reduce the need for wave soldering—a step prone to defects. 

In most factories, defect levels at the wave step are higher than those for the oven mass reflow process. The defects are related to poor process setup, poor process control, inadequate PWB design, or any combination of the three. Although wave soldering has been around for a long time, it is stiU not very well understood, due mainly to varying machine configurations and number of process variables. 

Avoid double-sided wave-soldered PTH components Double-sided wave soldering is generally fraught with defects. Determine whether SMT or press-fit technology can be used to replace a second wave-soldering operation. The industry-standard preference is to restrict all wave-solderable components to one side (components inserted through the primary side) of the PWB. 

A common defect associated with no-clean wave soldering is the seepage of flux between the pallet and the board. The resulting flux residue may inhibit in-circuit test probe contact. Care should be taken to ensure that the board fits well into the pallet nest, is rigidly indexed within the nest of the pallet, and is adequately retained against the pallet surface. As a pallet ages, it may shrink, take on twist, or bow and even delaminate. All of these may interfere with proper board seating. 

Pb-free wave-solder joint appearance snch as dniler solder joints Just as in reflow soldering, wave-soldered joints tend to be duller than those obtained from eutectic Sn-Pb.This is not a defect but a reality. 

SPC requires reliable data that can be analyzed either in real time or historically. Visual inspection collects defect data, such as the number of solder joint defects per assembly right after the solder reflow process (either reflow or wave soldering). Some manual and automated inspection techniques also take quantitative measurements of key assembly parameters, such as solder paste volume or solder joint fillet height. To the extent that these data are repeatable, manufacturers use defect data or measurements to characterize the amount of process variation from assembly to assembly or from solder joint to solder joint. When the amount of variation starts to drift outside its normal range or outside its control limits, manufacturers can assess the assembly process and monitor or choose to take action until the process is adjusted to eliminate this drift. Historical analysis of the defect or measurement... 

Visual inspection after the reflow and wave-soldering process steps can also be jnst a qnick scan for obvious defects to detect a process condition outside of control limits. In this case, the operator visually inspects for solder bridges, large solder balls or solder splashes, lifted leads, and a... 

The wave soldering process has been precisely engineered for Pb n eutectic solder. The type of flux, the take-off angle of the assembly as it exits the wave, the horizontal (belt) speed, and the shape of the wave, have all been specifically engineered to minimize skips and bridges, the common defects associated with wave soldering. 

In eutectic Sn-Pb wave soldering, formation of solderballs on the surface of a PCB is a low-occurrence defect. For lead-free wave soldering, however, the number of solderball defects is substantially higher than with eutectic Sn-Pb. The higher operating temperatures required in lead-free soldering softens the solder resist on boards which makes solderballs more prone to stick to the board surface. In addition, the reduced wettabihty and increased oxide formation found with many lead-free solders increases the chance of solderball formation. 

Fig. 36 shows the evaluation board used to study the lead-free wave soldering process, and Table 7 lists the components and their finishes addressed in these studies. A comparative defect summary for several proposed lead-free solders was prepared (Table 8) as a result of the evaluation conducted, as were joint strength measurements (Table 9). Several observations were made based on the studies that are listed in Table 10. 

Panasonic has experienced similar results when lead-free solder assembly was phased in for TV production. The defect levels dropped from a historical rate of 3% with eutectic Sn-Pb to a 0.07% level with Sn-3.5Ag-0.5Bi-8In solder paste and Sn-3.0Ag-0.5Cu wave solder. 

FIG. 62 Additions of trace amounts of Ni to the Sn-0.7Cu bath have shown to dramatically improve the wave solder bath by reducing the dross rate, stabilizing the wave shape and height, and eliminating whisker defects. (From Ref. 24.)... 

Mass-production wave soldering began on one production site in mid-1998 with Sn-5Bi-2Sb-lAg solder on single-sided printed boards with a wide variety of surface-mounted and through-hole-mounted components. The wave soldering process is very stable and has a first-pass defect rate of only 30 ppm. Industrial reflow soldering began on another site in early 1999, with no details available about process stability or yield. 

Dispensing processes include those for an adhesive, solder paste, or both (e.g., surface-mount wave or selective soldering). Because the adhesive is dispensed prior to the solder paste, it is possible to inspect for defects—incorrect dot volume, run-out on nearby solder pads or lands, and stringers—prior to dispensing solder paste. 

Shorts between solder pins Wave/reflow soldering Too much solder, solder screen defect, pin misregistration, bent pins... 

Solder open Solder application, wave/ reflow soldering Too little solder, solder screen defects, tombstoning, bent pins... 

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