In the SMT process of mixed-assembly boards, if surface-mount components are reflowed using Solder Paste printing while through-hole pins rely on manual Solder Wire touch-up soldering, without synchronously controlling the matching relationship between the solder paste melting point and the activation temperature of the solder wire Flux, a phenomenon will occur at the end of the preheating zone where some solder paste has not yet fully wetted while manual solder joints have already solidified in advance. This results in insufficient thrust of through-hole solder joints and makes internal voids difficult to detect via X-ray inspection. The root cause is that most engineers apply the solder paste reflow profile as a fixed template, ignoring that the continuous solder supply process of Sn63/Pb37 solder wire after it starts melting at 183°C will cause thermal interference with the volatilization stage of solder paste residual solvents. The correct approach is to lower the reflow peak temperature by 15°C and extend the time above liquidus to 60 seconds, ensuring that the solder paste completes metallurgical bonding before initiating manual touch-up soldering.

The stencil aperture dimensions are designed according to pure SMT standards without stepped narrowing treatment for through-hole pads in the mixed-assembly area, causing excessive solder paste accumulation on through-hole pads around QFP-type devices. After reflow, solder balls form and are blown by airflow to adjacent pins, causing bridging. This phenomenon is particularly prominent in areas where 0.4mm pitch QFPs coexist with adjacent Φ1.0mm through-hole components. The root cause is that the process documents do not clearly distinguish the stencil compensation rules between 'surface-mount pads' and 'mixed-assembly pads'. The correct approach is to implement -12% area reduction for all SMT pads adjacent to through-hole components, and add a 0.05mm thick nickel layer on the stencil bottom to suppress lateral solder paste spreading.

Manual touch-up soldering operators are accustomed to directly applying solder wire to uncooled PCBs at the reflow oven exit. At this time, the PCB board surface temperature is still as high as over 110°C, and the rosin flux in the solder wire carbonizes at high temperature and remains on the pad edges. During subsequent ICT testing, probe contact impedance fluctuations exceed 3Ω, and in severe cases, false open-circuit judgments are induced. The essence of the problem is the misunderstanding of 'operable temperature window' as 'can be operated when not hot to the touch'. The correct approach is to configure an infrared temperature gun to forcibly monitor the board surface temperature, and manual solder wire soldering must only begin after the PCB naturally cools to ≤40°C, with continuous heating at a single point not exceeding 3 seconds.

Using both ROL0 and ROL1 type solder pastes on the same mixed-assembly board while pairing with 63/37 solder wire containing halogen activators leads to gray-white corrosion spots on the solder joint surfaces after reflow, and creeping corrosion of copper pads occurs within two weeks. This is because the ion content of ROL0 solder paste residue is <0.5μg/cm², which cannot neutralize the erosion effect of Cl⁻ in the solder wire flux. The wrong perception is that 'solder paste covering the pad equals completing the cleaning closed loop'. The correct approach is to restrict mixed-assembly boards to only allow the combination of ROL1 grade solder paste and halogen-free solder wire, or add a local water washing process after Manual Soldering, with the water washing conductivity controlled at ≤10μS/cm.

To meet production output targets, through-hole touch-up soldering is concentratedly completed in batches within 30 minutes after reflow, causing the PCB to repeatedly experience thermal cycle shocks from 200°C → room temperature → 250°C. Delamination and blistering of the FR4 substrate occur in the third-layer signal routing area on the back side of the BGA. The failure mechanism is molecular chain breakage after the epoxy resin glass transition temperature Tg (135°C) is repeatedly exceeded, rather than simply excessive soldering temperature. The real problem is that the production scheduling does not reserve a ≥90-minute thermal equilibrium standing period for mixed-assembly boards. The correct approach is to physically isolate the manual soldering station from the reflow oven by at least 5 meters, and set up a buffer workstation with constant temperature and humidity at 25±2°C / 55±5%RH.

Checklist: Confirm whether -12% area compensation is implemented for stencil mixed-assembly area pads; verify whether the reflow profile peak temperature is 15°C lower than the standard value and the time above liquidus is ≥60 seconds; spot-check whether there are gray-white residues or carbonization traces around manual solder joints; measure whether the PCB board surface temperature before touch-up soldering is ≤40°C; verify whether the solder paste and solder wire are both ROL1 grade or both halogen-free systems.