Wave soldering 0603s: these small parts can usually withstand higher soldering temps.

The introduction of Pb-free alloys to wave soldering has made soldering more difficult when smaller components are used. Different techniques are employed for soldering small components such as 0603 resistors. In many cases, reflow soldering will be used. Components in 0603 sizes can be found in cellphones, hard disk drives, computer-related equipment, car radios, and many other products. These small components can generally withstand higher soldering temperatures. Typical maximum conditions are 10 sec. in liquidus solder of 260[degrees]C.

Two major requirements for good soldering conditions are the same as one would expect for any solder joint:

* Thermal solderability.

* Clean surfaces of the parts to be connected.

For wave soldering 0603s, this includes clean component and board finishes. The 0603 finish for Pb-free applications is most likely pure tin. If the components have been properly stored according to the defined MSL rating conditions, solderability of the component surfaces will not be a big deal. In practice, Pb-free board finishes have more solderability issues. Some processes have multiple reflow/curing cycles before the assembly is wave soldered.

These thermal processes may result in poor solderability due to oxidation. For example, copper OSP and immersion tin finishes are sensitive to multiple soldering processes. Flux used in wave soldering should remove the oxides. Also, a nitrogen blanket over the liquidus solder may help improve wetting. High-activity flux residues may weaken 0603 performance or reliability.

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Typical defects that may occur when soldering these small components include bridging open solder joints, solder balls between 0603s, and bulbous solder joints (Figure 1).

The flux is likely to have a major impact on the number of bridges. Nitrogen may support the flux in reducing the number of bridges. The glue dots must be accurately placed and the wave controlled to prevent bridging.

Debridging with a debridging tool in lead-free wave soldering may remove the bridges between 0603s, but the process window is small since the solder solidifies more quickly than SnPb in a leaded wave soldering process. Open solder joints may have different causes. The poor solderability of the board finish is one possible reason. It is recommended that thicker or more even plating be achieved during the hot air leveling process at the printed circuit board manufacturing stage, to help eliminate no/skipped solder defects.

Other possible causes for open joints are chip wave contamination or a lack of flux at that part of the assembly. Preventive maintenance of the chip wave and the spray nozzle head will help increase yields.

Solder balls result from poor flux activity and can also be improved by selecting a proper solder resist. A bulbous solder joint doesn't need to be reworked, provided the solder is only touching the metal surface of the 0603 component. This is more of a cosmetic issue than a defect.

Design recommendations. The pad design for a wave soldered 0603 component may slightly differ from a reflow soldered one; the dimensions shown (Figure 2) can be used for both processes.

Small 0603 SMDs can also be soldered successfully with Pb-free alloys at higher temperatures. With a proper design (pad dimensions) and proper storage, the results will not be much different from those of the SnPb process. The board finish must be free of oxides to achieve proper wetting.

Detailed information about reflow soldering 0603 SMDs can be found in the NPL report DEPC-MPR 044. The paper also goes into detail on challenges when wave soldering small components.

Gerjan Diepstraten is a senior process engineer with Vitronics Soltec BV (vitronics-soltec.com); gdiepstraten@nl. vitronics-soltec.com. His column appears monthly.

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