Solder paste stencil printing defects are a high-frequency failure in SMT production lines. Essentially, they occur when solder paste fails to transfer completely onto PCB pads. The root causes are often intertwined with stencil aperture accuracy, tension degradation, and the rheological properties of the solder paste. It is necessary to simultaneously inspect stencil wear status, squeegee pressure parameters, and changes in solder paste viscosity. Neglecting any of these aspects will lead to false yield fluctuations.

Eco-friendly solder wire is not equivalent to RoHS compliance. It is necessary to confirm whether the content of six hazardous substances, including lead, mercury, and cadmium, is below the limit through a third-party test report. A supplier's verbal commitment or "lead-free" label alone cannot replace actual test data. Production lines often overlook batch differences, resulting in typical RoHS non-compliance symptoms such as dull solder joints and poor wetting after the entire batch of PCBs goes through the reflow oven.

Severe oxidation of solder bars directly leads to solder joint spikes, cold solder joints, and increased solder dross. Intervention must be carried out simultaneously from four aspects: storage environment, solder melting temperature, flux ratio, and squeegee maintenance. Among them, controlling the solder pot temperature at 250±5℃, using special flux containing antioxidant components, and keeping solder bars dry and sealed during storage are the three most effective practical measures to reduce solder oxidation.

Solder paste caking is a high-frequency failure on SMT production lines. The main causes include uncontrolled storage temperature and humidity, repeated freeze-thaw cycles, excessive exposure after opening, and uneven stirring. Caked solder paste has undergone metal particle agglomeration and flux phase separation, and its rheological properties cannot be restored even after re-stirring. Forced use will lead to defects such as missing printing, insufficient solder, and bridging, and it must be scrapped.

The tensile strength and elongation of solder wire directly determine the stability and workability of soldering performance. Insufficient tensile strength easily causes wire breakage and jamming during automatic wire feeding, while inadequate elongation leads to difficulties in dragging solder during manual soldering and solder joint spikes. The two jointly affect SMT pre-tinning, wave soldering bridging control, and rework yield.

When purchasing solder bars, solder purity directly affects soldering reliability and production line yield. The quality of solder bars can be quickly preliminarily assessed by observing the fluidity in the molten state, the gloss of the fracture surface after cooling, the scraping feel, and the differences in flux reaction. Combined with on-site scanning using an XRF detector, the content of impurities such as lead, copper, and iron can be accurately identified, avoiding cold solder joints, poor wetting, or PCB corrosion caused by insufficient purity.

The proportion of flux in solder paste is usually controlled between 8% and 12%. This range can not only ensure the viscosity and release performance during printing, but also meet the dual requirements of active release and controllable residue during the reflow stage. Too high a proportion can easily lead to slump and splashing, while too low a proportion can cause poor wetting and cold soldering. The actual production line needs to be dynamically fine-tuned in combination with stencil apertures, ambient temperature and humidity, and reflow oven capabilities.

There are significant differences between silver-containing solder wire and ordinary solder wire in solder joint wettability, mechanical strength, and thermal fatigue life. Silver-containing solder improves the tin climbing height because silver reduces the surface tension of the molten solder. Meanwhile, the silver-tin intermetallic compound is denser, increasing the shear resistance of solder joints by more than 15%. The selection of solder wire must be comprehensively determined based on PCB copper thickness, component pin plating, and reflow peak temperature.

Whether solder paste contains halogens is directly related to the long-term reliability and environmental compliance of PCBA. Halogen-free solder paste requires the total halogen content to be below 900ppm. Detection needs to combine ion chromatography and sample pretreatment processes. Production lines often suffer from misjudgments due to neglecting extraction temperature control or errors in standard solution preparation.