Transitioning to lead-free assembly electronics requires not only product reliability, but also the ability for an optimized PCB assembly process. Additionally, material selection, component changes, and handling potential testing failures are integral to a successful transition.
Surface finish is one of the key material choices impacting the entire assembly electronics process. The assembly process itself, plus the complexity of the board and its use environment, all make an impact on this decision. Typical options include lead-free HASL or ENIG, OSP, ImAg, and ImSN, with new innovations becoming market-ready all the time. Different laminates are often under-specified due to underestimating their quality during the assembly process. Alternatively, laminates may be over-specified, which commands a higher cost.
Solder alloys are another important decision: wave solder, surface mount, and solder for BGA balls all have to be considered. Typ[ically, SAC305 is the surface-mount alloy of choice, and it performs acceptably under most circumstances. Depending on the complexity required for your wave solder alloys, the most cost effective solution is SnCU, especially if you have thin boards with low-aspect holes that need to be filled. SnAgCuNi is preferable for more challenging holes, although this may depend on the surface finish and choice of flux as well. For board flexing capabilities, or withstanding shock events, a variety of lower modulus alloys like SAC105 are becoming more prevalent.
Component Qualifications and Testing
Early adopters of lead-free assembly electronics discovered frequent occurrences of melted plastic components on wave solder connectors. This lead to a switch away from nylon 66 toward the more appropriate nylon 46, or sometimes LCP. These meet the requirements of moisture and temperature sensitivity, a must-have in surface-mount LF components.
Other components that need to withstand heat damage may be more difficult to detect, which makes preventing that damage a greater challenge. Electrolytes within electrolytic capacitors may boil if time limits and safety temperatures are exceeded, yet these limits are not always outlined in sufficient detail.
Common specs may offer some suggestions, yet without the necessary variables to predict possible behavior. Component engineers end up guessing at preheat durations and total peak heat, while attempts are made with lead-free alloys to properly fill holes under various conditions.
Testing the strain capability of separate components following lead-free assembly is another blind spot. For example, it’s commonly known that SAC alloys have a reduced tolerance for strain, yet their assembly manufacturing people rarely offer specific limits for these devices. If suppliers could record results of four-point bend testing for their components in order to determine an objective maximum strain capability, the user could know if problems are more likely to arise during component stuffing, performing ICT, or attaching their boards to the chassis.
All too often, lead-free reworks are overlooked in favor or meeting tight manufacturing schedules. However, records indicate that reworking is one of the greatest challenges involved with making the transition to lead-free assembly. A Lead-free assembly electronic has a more difficult window lying between the cold solder on one end and heat damage at the other.
Reworking can bring operator-to-operator benefits to the mix, optimizing equipment process that was probably originally designed for Sn-Pb rework. Every critical component requires a strict rework process which must be rigidly adhered to. Extensive failure analysis has to be undertaken before a determination regarding appropriate rework procedure can be made. Damage to the PCB, the component itself, or any neighboring components must be prevented. New rework equipment may be necessary, as well as the employment of new shielding techniques.
Despite the growing experience with assemblage of lead-free PCBs, high quality is not always maintained after high-volume production begins. A comprehensive audit plan should be in place to address with the supplier in order to ensure accountability. Making the change to lead-free assembly electronics provides an excellent opportunity to review your supplier’s typical process, and make any necessary adjustments for optimum quality control.