The conveyor, cooling system and reflow atmosphere are critical factors.

The assembly yield of a specific PWB will depend on the interaction of two variables: materials and process. Materials employed in construction of electronics products are being changed in such a fashion as to have a significant effect on the assembly process. When focusing on any assembly process, especially the soldering process, it is critical to identify changes these variables undergo to understand and consequently improve yield and reliability while considering the “cost of assembly.”

In a reflow soldering process, the major challenges are the use of alternative, Pb-free alloys and board finishes; conversion to and use of RoHS-compliant components; and selection of appropriate laminate materials. These material changes require a reflow soldering oven capable of providing and controlling a specific process characterized by a significant reduction in the process window as compared to the classical SnPb process. Higher peak temperatures and longer time above liquidus are required. These operating conditions will subject the boards, components and fluxes to their respective limits. This column breaks down the process into systems and identifies the tradeoffs required.

Some equipment requires slowing the conveyor speed to maintain temperatures within the process window. This, in return, reduces the throughput capacity, increases the risk of component overheating, and produces thicker intermetallics in the solder joints. Components must be classified to an appropriate moisture sensitivity level (MSL) to avoid damage during the reflow process. The rapid heating of the moisture absorbed within the plastic components quickly turns into superheated steam, which produces pressure within the package, resulting in delamination or cracking. Surface mount non-hermetic components must follow J-STD-020C to be compatible with Pb-free processes. Depending on the component size, this standard requires maximum reflow temperatures up to 260°C for 40 sec.

Decreasing conveyor speed will also increase the TAL. This increase may result in charred flux residues and formation of thicker intermetallics (Figure 1). Thicker intermetallics are brittle areas where solder joints can crack easily during normal operation. To avoid this serious problem, reflow ovens with better heat transfer are required to maintain higher conveyor speeds.


Another important issue that must be considered is conveyor robustness. Higher reflow temperatures can affect its parallelism. In this scenario, boards may drop. Conveyors should be fitted with a single or dual support system to avoid dropping thin boards that may warp during heating. The inclusion of a mesh belt in the conveyor system is another option.

Flux vapor management is an important issue to consider. Flux behavior is affected by profile type and brand. Flux will evaporate on the soak and peak process stages, and even in cooling zones of reflow ovens. Flux vapor management systems track thermal performance, reduce preventive maintenance, and also control flow and removal of flux evaporation. Typical systems remove gases through slotted exhaust ports in each cell of the oven and direct them away through a stack filter. Gas recirculation systems that collect and filter gases while controlling contamination are also available.

The cooling gradient is defined by the most critical components in the assembly. A rapid cooling rate may result in component cracking and a very slow cooling rate in thicker intermetallics. Cooling at the rate of 2-3°C/s will produce homogeneous grain structure and good solder joint reliability. The cooling rates depend on the peak temperature, the TAL, the desired board exit, the assembly thermal mass, and the maximum allowable cooling gradient (6°C/s). Adequate reflow ovens should be able to provide users with efficient cooling systems that will achieve desired cooling rates.

Finally, the appropriate reflow atmosphere must be analyzed. Heating solder in an air oven will create oxides. The flux in the solder paste should be strong enough to remove oxides and protect the surfaces of the components and the board. Using nitrogen can weaken these fluxes. The necessity of nitrogen in soldering is often discussed. Some processes require nitrogen to improve wetting, especially when there are oxidation issues due to material mishandling, and others may result in more tombstoning defects or no benefit.

The question still remains as to whether the nitrogen cost is justified. An inert atmosphere should be avoided because of cost, but in instances of new technology (smaller and more complex components and boards), one always should have the ability to switch to a nitrogen environment. Every process has its specific issues and challenges. It is necessary to review nitrogen performance and necessity after implementing Pb-free soldering.

After longer production periods, one is able to decide whether the decision was correct. It is, therefore, a good choice to have an oven that can switch easily from air to nitrogen.

Reflow ovens are set up to achieve <100 ppm throughout the tunnel. The more nitrogen used, the higher the system lifecycle cost. To reduce nitrogen consumption, ppm levels should be optimized and nitrogen applied only in critical zones of the reflow oven: end of soak zone, peak zones and first cooling zones where the assemblies are sensitive to oxidation. The optimal ppm levels depend on the application. An internal study¹ suggested ppm level up to 2500 ppm.

References

1. U. Marquez, D. Barbini and W. Enroth, “Impact of Soldering Atmosphere on Solder Joint Formation,” SMTA Pan Pacific Symposium, January 2008.

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.