Circuits Assembly - October 2008 - (Page 34)

Reflow Soldering Is My Oven Pb-Free Capable? The conveyor, cooling system and reflow atmosphere are critical factors. affect its parallelism. In this scenario, boards may drop. he assembly yield of a specific PWB will depend Conveyors should be fitted with a single or dual support on the interaction of two variables: materials and system to avoid dropping thin boards that may warp process. Materials employed in construction of during heating. The inclusion of a mesh belt in the conelectronics products are being changed in such a fashion veyor system is another option. as to have a significant effect on the assembly process. Flux vapor management is an important issue to When focusing on any assembly process, especially consider. Flux behavior is affected by profile type and the soldering process, it is critical to identify changes brand. Flux will evaporate on the soak and peak prothese variables undergo to understand and consequently cess stages, and even in cooling zones of reflow ovens. improve yield and reliability while considering the “cost Flux vapor management systems track thermal perforof assembly.” mance, reduce preventive maintenance, and also control In a reflow soldering process, the major challenges are flow and removal of flux evaporation. Typical systems the use of alternative, Pb-free alloys and board finishes; remove gases through slotted exhaust ports in each cell conversion to and use of RoHS-compliant components; of the oven and direct them away through a stack filter. and selection of appropriate laminate materials. These Gas recirculation systems that collect and filter gases material changes require a reflow soldering oven capable while controlling contamination are also available. of providing and controlling a specific process characThe cooling gradient is defined by terized by a significant reduction in the most critical components in the the process window as compared to assembly. A rapid cooling rate may the classical SnPb process. Higher result in component cracking and peak temperatures and longer time a very slow cooling rate in thicker above liquidus are required. These intermetallics. Cooling at the rate of operating conditions will subject the 2-3°C/s will produce homogeneous boards, components and fluxes to their respective limits. This column grain structure and good solder joint breaks down the process into systems reliability. The cooling rates depend and identifies the tradeoffs required. on the peak temperature, the TAL, Figure 1. Intermetallics in a solder Some equipment requires slowing the desired board exit, the assembly joint – etched sample. the conveyor speed to maintain temthermal mass, and the maximum peratures within the process window. allowable cooling gradient (6°C/s). This, in return, reduces the throughput capacity, increasAdequate reflow ovens should be able to provide users es the risk of component overheating, and produces with efficient cooling systems that will achieve desired thicker intermetallics in the solder joints. Components cooling rates. must be classified to an appropriate moisture sensitivity Finally, the appropriate reflow atmosphere must be level (MSL) to avoid damage during the reflow process. analyzed. Heating solder in an air oven will create oxides. The rapid heating of the moisture absorbed within The flux in the solder paste should be strong enough to the plastic components quickly turns into superheated remove oxides and protect the surfaces of the composteam, which produces pressure within the package, nents and the board. Using nitrogen can weaken these resulting in delamination or cracking. Surface mount fluxes. The necessity of nitrogen in soldering is often non-hermetic components must follow J-STD-020C to discussed. Some processes require nitrogen to improve be compatible with Pb-free processes. Depending on the wetting, especially when there are oxidation issues due component size, this standard requires maximum reflow to material mishandling, and others may result in more temperatures up to 260°C for 40 sec. tombstoning defects or no benefit. The question still remains as to whether the nitroDecreasing conveyor speed will also increase the TAL. gen cost is justified. An inert atmosphere should be This increase may result in charred flux residues and avoided because of cost, but in instances of new techformation of thicker intermetallics (Figure 1). Thicker nology (smaller and more complex components and intermetallics are brittle areas where solder joints can boards), one always should have the ability to switch to crack easily during normal operation. To avoid this seria nitrogen environment. Every process has its specific ous problem, reflow ovens with better heat transfer are issues and challenges. It is necessary to review nitrogen required to maintain higher conveyor speeds. performance and necessity after implementing Pb-free Another important issue that must be considered is soldering. conveyor robustness. Higher reflow temperatures can Continued on pg. 47 Circuits Assembly OCTOBER 2008 circuitsassembly.com T 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. 34 http://www.vitronics-soltec.com http://www.circuitsassembly.com

Table of Contents Feed for the Digital Edition of Circuits Assembly - October 2008

Circuits Assembly - September 2008 Contents Letters Caveat Lector Industry News Market Watch Talking Heads Global Sourcing Screen Printing Better Manufacturing 'Checking Up' on Medical Electronics Solder Ball Attachment Using Laser Soldering Improving QFN Reliability Reflow Soldering Tech Tips Test and Inspection Process Doctor Pb-Free Lessons Learned Alternative Energies Eastern Advances Product Spotlight Ad Index Assembly Insider Technical Abstracts

Circuits Assembly - October 2008

For optimal viewing of this digital publication, please enable JavaScript and then refresh the page. If you would like to try to load the digital publication without using Flash Player detection, please click here.