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Lead-free solder paste is a paste-like soldering material made by uniformly blending lead-free solder alloy powder with a flux medium, used for SMT surface mounting. SAC305 is the most common lead-free alloy, composed of 96.5% tin, 3.0% silver and 0.5% copper (Sn96.5Ag3.0Cu0.5), with a melting point of about 217C. Compared with leaded Sn63Pb37 (melting point 183C), it is more eco-friendly and produces stronger joints, meeting the EU RoHS and REACH requirements.

Voids are gas cavities inside a solder joint visible under X-Ray. They are mainly caused by flux volatiles failing to escape during the liquid phase of reflow, oxidation of pads or powder, or improper stencil aperture design, and are especially pronounced on QFN thermal pads and large BGA spheres. Improvements include optimizing the reflow profile, extending the soak to let volatiles escape, using vacuum reflow, optimizing stencil apertures (grid/divided windows), and strengthening paste cold-storage, warm-up and mixing management.

Grape clustering is a defect in which solder powder fails to fully coalesce after reflow, forming grape-bunch-like particles at the component solder joints. It is usually caused by paste oxidation, insufficient flux activity, inadequate peak reflow temperature or time above liquidus, or moisture absorption, and occurs mainly at the joints rather than across the whole device. Countermeasures include increasing flux activity, raising peak temperature and time above liquidus, and controlling paste storage and the operating environment.

Head-in-pillow (HIP) is an open defect in which a BGA sphere appears to rest on the paste but fails to coalesce with it after reflow, resembling a head on a pillow. It is mainly caused by warpage of the BGA package or PCB during reflow, oxidation of spheres or pads, insufficient flux activity or an improper temperature profile. Improvements include optimizing package and PCB design to reduce warpage, baking before soldering to remove moisture, using nitrogen reflow, increasing flux activity, optimizing ramp and peak temperatures, and verifying with X-Ray inspection.

Leaded solder, typified by Sn63Pb37, melts at 183C with good wetting and low cost, but lead is toxic and not eco-friendly. Lead-free solder, typified by SAC305, melts at about 217C, complies with RoHS/REACH and offers stronger, more fatigue-resistant joints, but requires higher soldering temperatures and tighter process control. Lead-free solder is generally required for export, consumer electronics and automotive electronics.

Selection should consider component temperature tolerance (SAC305 for standard components; low-temperature Sn-Bi alloys such as Sn42Bi58, melting around 138C, for heat-sensitive parts), printability and wetting, reliability requirements, environmental certifications (RoHS, REACH, halogen-free) and cost. HOERSON provides total technical support for alloy selection, sample evaluation and reflow profile optimization.

Solder paste should generally be refrigerated at 0-10C in a sealed container. Before use, let it warm to room temperature while still sealed (about 4-6 hours); never open it during warm-up, to avoid moisture absorption. Mix thoroughly before use. The printing environment should be 20-25C with 40-60% relative humidity. Use within the specified time after opening, avoiding prolonged exposure that causes oxidation and performance degradation.