Lead-free wave soldering

There are several important decisions to make when converting to lead-free wave soldering. This article details the feasibility of purchasing new equipment vs. using existing wave-solder equipment for lead-free conversion. 

The preheat temperature for lead-free wave soldering is not very different than that used with tin/lead soldering. The typical top-side temperature for lead-free wave soldering ranges from 180° to 225°C, depending on the flux in use. 

The implementation of lead-free wave soldering brings with it several decisions. Whether to purchase new equipment, which flux to implement, and which solder alloy to use can have a significant impact on the throughput of your operation and, ultimately, your bottom line. As always, no one can tell you what is best for your business. 

Lead-Free Wave Solder Alloy Selection Reliability is Key... 

In an ideal world, there would be a lead-free wave solder alloy that gives as good or better overall performance as Sn63/Pb37— a drop-in replacement. Unfortunately, this does not exist. What is obtainable in the market today are various lead-free wave solder alloys that possess different properties. For PCB assemblers, this means that they need to make a choice of alloy and for many, this means that they need to conduct extensive in-house testing to make an informed choice. 

For a lead-free wave soldering process, a three zone (length about 1800 mm) preheat region is sufficient to heat the boards and remove flux solvents. A typical wave solder preheat configuration for lead-free solders would consist of calrods for the first zone, followed by forced convection in zones two and three. 

VIII. LEAD-FREE WAVE SOLDERING A. Double-Wave Soldering... 

SMD-component, lead-free wave soldering is that components experience a solder immersion temperature that exceeds 250°C which may result in component damage. 

Similar to traditional Sn-Pb wave soldering, metals dissolve relatively quickly during lead-free wave soldering operations. The rate of alloy or metal dissolution of exposed pads on PCBs or component leads depends upon their composition, solder composition, bath temperature, and the flow velocity of the wave solder. The rate of dissolution of a specific metal is lower if this metal is already present in the lead-free solder bath. Of particular concern is the dissolution of copper from circuit boards when exposed to lead-free solder waves. 

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. 

FIG. 37 Thermal cycle tests for a 0.8-mm pitch QFP using several lead-free wave solder alloys. Thermal cycling conditions were -40°C to 85°C for 30-min dwell times. Electrode finishes tested were a) Sn-Pb finish, b) Sn-Bi finish, and c) Pd finish. The Sn-Ag-Cu lead-free alloy is generally observed to be the most resilient to electrode finish materials, outperforming even the eutectic Sn-Pb solder in most cases. (From Ref. 24)... 

FIG. 52 A real-time monitor based on DTA techniques was developed in conjunction with Malcom Industries to automatically monitor the impurity levels in a lead-free wave soldering bath, a) The monitor indicates the b) initial melting point of the solder bath alloy and c) lower melting point due to lead (Pb) contamination. (Courtesy of Panasonic and Malcom Industries.)... 

FIG. 65 Photograph of Panasonic engineers demonstrating the lead-free wave soldering system at the... 

FIG. 22 First mass-produced, lead-free wave soldered product, (a) Top side, (b) Bottom side. 

Lead-free wave soldering requires a 10 15°C higher solder bath temperature and longer contact times between boards and solder waves. 

Biglari, M.H. de Kluizenaar, E.E. Langeveld, P. Schwarzbach, D. Oddy, M. Lead-free wave soldering with VOC-free fluxes part I Ahoy development based on SnAgCu, SnBiAg and SnCu, and process aspects. Proc. IPCWorks 2000, Miami, Florida, USA, September 2000. 

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