Solder Paste Printing Is the First Gate Determining PCB Soldering Success or Failure

The matching degree between stencil aperture dimensions and PCB pads must be controlled within ±0.02mm; otherwise, 0201 components will experience solder bridging or insufficient solder. During spot checks using the JJY F-800 microscope, we found that for every 0.5kgf increase in squeegee pressure and 10mm/s decrease in printing speed, the fluctuation of solder paste thickness expands by 0.03mm, which directly leads to a 12% increase in subsequent placement offset rate. Especially when the warpage of FR-4 board exceeds 0.75mm, the solder volume attenuation of edge pads reaches 35%.

SMT Placement Process Must Simultaneously Constrain Nozzle Vacuum and Placement Force

The placement accuracy of 0.4mm pitch QFP devices depends on maintaining the nozzle vacuum degree within the range of –85kPa±3kPa. If the vacuum fluctuation exceeds the limit, the coordinate offset recognized by the placement machine's vision system will be superimposed with the repeat positioning error of the robotic arm, ultimately resulting in defective solder joints with a wetting angle greater than 30°. Statistics from the JJY production line show that such defects account for 68% of all cold solder joints. If the placement pressure exceeds 0.3N, it will squeeze the solder paste, leading to a fourfold increase in the risk of bridging.

Reflow Soldering Temperature Profile Is Not a Fixed Template but a Dynamic Response Parameter

Differences in heat capacity of PCBs at different positions in the same reflow oven can cause a measured peak temperature deviation of up to ±4℃. We strictly require that for every 1cm/s decrease in conveyor chain speed, the soak zone time must be extended by 8 seconds to compensate for heat conduction delay. When using lead-free solder paste, the time above liquidus must be strictly controlled within 60±5 seconds. If exceeded, the IMC layer thickness will exceed 3.2μm, causing solder joint brittle fracture; if insufficient, the carbonization rate of residual Flux will increase to 21%.

Wave Soldering Must Bind and Control PCB Solder Immersion Angle and Fixture Rigidity

When the solder immersion angle is set to 6°, the solder joint climb height is the most stable. However, if the fixture pressing depth is less than 0.15mm, the PCB will undergo elastic deformation at the 0.12mm level under wave impact, causing the solder filling rate in through-holes to drop sharply from 92% to 73%. During JJY's debugging, it was found that after the chain speed is increased to 1.4m/min, if the temperature of the PCB bottom surface at the preheating zone outlet is lower than 105℃, dense pinholes will appear on the solder joint surface, and the porosity detected by X-ray will exceed the standard by three times.

Manual Touch-Up Soldering Is by No Means Simple Heating but a Thermal Stress Redistribution Process

When performing local rework on a PCB with mounted BGAs, the distance between the hot air gun nozzle and the pad must be maintained at 22mm±1mm, with the air speed set to 1.8m/s, while using an infrared thermometer to monitor the surface temperature of adjacent ICs in real time. Heating must be paused immediately once the temperature exceeds 115℃; otherwise, after the glass transition temperature of the FR-4 substrate is exceeded, the pad peel strength will decrease by 40% irreversibly.