SJQ and SJR Study of SMT Soldering with Low Temperature Solder Paste

ABSTRACT This paper is in English and Chinese languages. The transition to Lead Free SnAgCu solders, has necessitated higher temperatures in reflow soldering processes, which has impacted manufacturing, from the aspect of cost, yield and rework. The manufacturing process windows have shrunk further with the introduction of thin packages and PCBs to support the demand for fancy consumer electronic products, such as tablets, smart phones, Ultrabooks® and wearable devices. This study was initiated with the intention of cost saving from lower soldering temperature and overcoming manufacturing challenges due to increased high warpage levels at the higher reflow temperatures of components, such as BGAs, connectors and PCBs. Common solutions applied in SMT manufacturing to overcome component and board high temperature warpage are customized stencil apertures to compensate the warpage gap by adding more solder paste to the identified locations, stiffening the PCB during reflow via customized reflow pallet, component baking prior to SMT and tighter screen printing and reflow process windows. Unfortunately, all these solutions incur additional manufacturing cost. In this study, we investigated the feasibility of SMT soldering of 0.6mm pitch SnAgCu ball FCBGAs with low temperature solder pastes, containing SnBi and SnAgBi (BSA) alloys. Bi containing solders have been shown previously to induce brittleness and potentially reduce solder joint reliability [1, 2, 3], and this was confirmed to some extent in the result of our reliability testing for mechanical shock. One of the alternatives to enhance the mechanical solder joint reliability of mixed SnAgCu-BiSn solder joints is to reinforce these solder joints with polymer resins that are contained within the solder paste (aka. Epoxy Flux, or Low Temperature solder paste with Resin Reinforcement, LTRR), and are cured during the reflow soldering process. This alternative was further investigated to first prove that the resin reinforcement did form around the solder joint after reflow soldering and to ascertain the improvement in solder joint reliability during mechanical shock and drop testing by this resin reinforcement [4, 5, 6]. This paper will present the SMT process details, solder joint yields and mechanical reliability results of mixed SnAgCu-BiSn based resin reinforced solder joints and compare them with the standard SAC solder joints and mixed SnAgCu -BiSnAg solder joints without any resin reinforcement.