Solder paste and red glue processes can be mixed on the same PCB, but the physical isolation between the red glue dispensing position and the solder paste printing area must be strictly controlled, the reflow soldering temperature profile must take into account both the red glue curing and solder paste melting windows, and the stencil aperture design, dispensing parameters and AOI inspection strategy must be collaboratively optimized. Otherwise, it is extremely easy to cause batch defects such as cold solder joints, tombstoning or red glue contamination of pads. This is a high-risk but implementable special process combination in SMT processes.

Whether solder bars can be mixed depends on the compatibility of alloy compositions and the matching degree with the process window of the wave soldering machine. Mixing different alloy solders can easily lead to risks such as melting point shift, uneven wetting, and embrittlement of solder joints. In actual production lines, common phenomena include increased solder dross, solder joint spikes, or cold solder joints. It is recommended to give priority to using solder bars from the same manufacturer and the same batch to ensure the stability of the alloy solder composition and the consistency of soldering.

Flux paste and solder paste have completely different functional roles in SMT placement soldering. Flux paste contains no metal powder and has no forming capability, so it cannot replace solder paste to achieve solder joint bonding and structural support. It can only serve as an auxiliary supplement to solder paste or a flux reinforcement method for local rework.

Excessive flux paste application is a high-frequency cause of component displacement in SMT production lines, and this issue is particularly prominent when soldering tiny components such as 0201 and 0402 packages. Based on 20 years of on-site production line measurement experience of JJY, this paper focuses on the coordinated adjustment of three key factors: squeegee pressure, stencil aperture design, and printing speed. It proposes an immediately executable parameter window calibration method to directly target the process balance point between flux paste and component soldering stability.

Lead-free solder paste and leaded solder paste have essential differences in composition, melting point, wettability, and reflow profile settings. The key to distinguishing them lies in the RoHS marking on the solder paste batch label, the color of flux residue, and the glossiness of solder joints. Since lead-free solder has a high melting point and high surface tension, it is necessary to increase the peak temperature and extend the time above the liquidus, while imposing higher requirements on stencil aperture design, squeegee pressure, and printing speed.

Although solder paste, solder cream, and solder wire all belong to electronic soldering consumables, their flux composition design logic is completely different. Solder paste emphasizes high viscosity and thixotropy to support precision dispensing; solder cream requires a balance of printability and reflow activity, adopting a combination of medium rosin content and volatile solvents; solder wire relies on low-solid-content flux in the core to achieve continuous supply and rapid volatilization. The three cannot be used interchangeably.

The size of the rosin-core hollow in solder wire directly affects the flux release amount, wetting speed and residue distribution. An excessively large hollow easily causes solder spatter, cold solder joints and rosin overflow that contaminates the PCB, while an excessively small hollow leads to insufficient flux, poor wetting and solder joint spikes. In actual production lines, 0.5mm diameter solder wire paired with a 0.12mm rosin-core hollow is the optimal combination for SMT manual touch-up soldering and rework.

A dull surface on solder wire does not necessarily indicate poor quality; it requires comprehensive judgment based on solder appearance, flux residue, storage environment, and degree of oxidation. For some low-smoke lead-free solder wires, natural dullness due to high rosin content or low-temperature storage is normal, while persistent blackening and reduced adhesion indicate flux deterioration or abnormal metal composition.

Solder paste printing forming spikes is a high-frequency fault on SMT production lines. The root causes are concentrated in three aspects: mismatch of solder paste rheological properties, unreasonable stencil aperture design, and unbalanced coordination between squeegee parameters and printing speed. The solution requires simultaneous adjustment of solder paste viscosity grade, optimization of squeegee pressure and angle combination, and calibration of stencil support tension and PCB positioning accuracy.

Solder paste mixing is a key process in the SMT production line that directly affects printing yield. Insufficient solder paste mixing will lead to abnormal viscosity, metal particle settlement, and flux stratification, which in turn cause typical defects such as missing printing, insufficient solder, and solder balls. In actual production lines, more than 65% of first-article defects are directly related to out-of-control solder paste mixing parameters.

The temperature control range of solder bars in wave soldering machines is typically 250°C to 265°C, which should be comprehensively adjusted based on the alloy composition of the solder bar, PCB thickness, component density, and flux activity. Excessively high wave soldering temperatures can easily cause copper foil peeling and thermal damage to components, while excessively low temperatures can lead to cold solder joints, solder spikes, and bridging. In actual production lines, dynamic fine-tuning must be performed by combining solder fluidity tests and solder joint cross-section analysis.

Silver-containing solder wire and SAC305 solder paste can achieve stable soldering on actual SMT production lines, but the reflow temperature profile and flux activity must be strictly matched; otherwise, problems such as poor wetting, increased void rate, and abnormal thickening of the IMC layer are likely to occur, with particularly sensitive performance on QFN and 0201 components. This combination has been verified as feasible in Jia Jinyuan's 20-year mass production cases, provided that the silver content of the solder wire is controlled at 2.0% to 3.2%, the metal content of the solder paste is maintained at 88.5% ± 0.3%, and the reflow peak temperature is locked at 235°C ± 2°C.

If solder paste is improperly selected or incompletely cleaned during PCB soldering, residual halide ions and moisture will act together to trigger electrochemical migration, leading to corrosion of copper circuits on the PCB, reduced insulation resistance, and severely compromised long-term reliability, with the failure risk rising significantly especially in high-temperature and high-humidity environments.

Whether expired solder paste can still be put into production depends on the actual storage conditions, compliance with temperature recovery procedures, and process verification results throughout the printing and reflow soldering process, rather than being simply determined based on the solder paste expiration label. In production lines, improper low-temperature cold storage often leads to sudden changes in solder paste viscosity, flux stratification, or oxidation of metal particles, which in turn causes batch defects such as insufficient solder, solder bridges, and cold solder joints.

Whether solder paste solidifies and fails during low-temperature storage depends on its formulation system and storage conditions. Common rosin-based solder paste tends to stratify or increase in viscosity below 5°C, but this does not mean complete failure. Its printing performance can be restored as long as it is returned to a constant temperature of 25°C for more than 4 hours and fully stirred. The key to solder paste storage is to avoid repeated freeze-thaw cycles and moisture intrusion.

Lead-containing and lead-free solder wires must not be mixed on the same production line. Due to the large difference in melting points, incompatible flux systems, and high risk of residue corrosion, mixing them will lead to cold solder joints, bridging, brittle solder joint cracking, and AOI misjudgment. The 217°C melting point of lead-free solder and the 183°C melting point of lead-containing solder wire create a thermal stress conflict, and their halogen contents and activation temperatures differ, which can easily cause batch soldering defects.

Excessive wave soldering dross directly affects soldering yield and the consumption cost of solder bars. Common causes include high impurity content in solder bars, uncontrolled wave soldering temperature, severe carbonized flux residue, excessive stirring of the solder pot, and insufficient nitrogen protection. Solutions require coordinated efforts across multiple dimensions, including solder bar selection, temperature profile optimization, regular dross removal frequency, flux volatilization control, and nitrogen purity monitoring.

The large amount of smoke generated by solder wire during welding mainly comes from thermal decomposition of flux, excessively high welding temperature, excessively high rosin content in the solder wire, or poor ventilation. These factors jointly increase the generation intensity and toxicity risk of welding fumes, which is particularly prominent in wave soldering and manual soldering iron operations.

Although solder wires, solder bars and solder paste all belong to the lead-free solder system, differences in their forms lead to different design logics for alloy composition. In actual production lines, mixing them easily causes problems such as poor wettability, increased void rate and whitening residue after reflow soldering. The alloy ratios of the three cannot be simply interchanged; dedicated formulations matching the corresponding form must be selected according to process requirements.

The division of application scenarios between solder paste and liquid flux depends on solder joint size, component density, and equipment compatibility. Solder paste is suitable for stencil printing or dispensing processes for 0201 and smaller micro components, while liquid flux is mostly used for through-hole insertion, rework on large solder pads, and pre-spraying before selective wave soldering. The two are often used in combination in SMT production lines according to process cycle time and residue control requirements.