The penetration rate of through-hole Reflow Soldering in the automotive electronics and industrial control fields has exceeded 18%. The driving factor is customers' rigid demand for BOM streamlining and assembly process compression, which forces production lines to abandon traditional wave soldering supporting fixtures and instead adopt a co-line production model with surface mount technology. However, for connectors with pin coplanarity deviation exceeding 0.15mm, the cold solder joint rate after reflow sharply increases to over 12%, and ordinary stencil openings cannot compensate for such mechanical tolerances. It is necessary to increase Solder Paste volume through stepped local thickening or back etching.
Solder paste selection is rapidly shifting from general-purpose to specialized types. Mainstream manufacturers have stopped supplying through-hole solder paste with particle sizes below T4, because T3 powder is prone to collapse and edge peeling in apertures above 0.5mm. Currently, leading EMS factories generally lock in T4-T5 mixed powder medium-high viscosity products with a viscosity range of 650–750 Pa·s. Under conditions of squeegee pressure 2.8–3.2kg and speed 45–55mm/s, this type of solder paste can maintain an in-aperture filling rate ≥88%, but its thixotropic index must be strictly controlled within the range of 0.62–0.68; otherwise, excessive rebound during stencil demolding will cause the entire solder paste to break.
Stencil design no longer relies on empirical look-up tables, but instead uses measured coplanarity data as input for reverse modeling. For example, the Z-direction distribution of pins of a certain domestic relay presents a bimodal shape with a peak spacing of 0.12mm. At this time, if a full-plate stencil is made according to the nominal thickness of 0.12mm, the actual solder amount on the low-side pins after printing is only 63% of the theoretical value. We have verified in three mass-produced models: implementing 0.06mm local thickening on the low-side area with a wide coverage of ±0.3mm around the pin centerline can narrow the solder amount difference on both sides to ±5%, while reducing post-reflow bridge defects from 3200ppm per million pieces to 490ppm.
Stencil opening shapes are shifting from rectangular to trapezoidal chamfers, because right-angle edges generate stress concentration during demolding, causing the top of the solder paste to tear and remain on the bottom surface of the stencil. After a certain German automotive-grade PCBA switched to 15° chamfered openings, the 24-hour continuous printing stability improved to CPK=1.42, and the cleaning frequency was extended from once every 4 hours to once every 11 hours. However, this optimization is only applicable to scenarios where the opening aspect ratio ≤3.5. Once the pin pitch is compressed below 0.8mm, the chamfer instead exacerbates the risk of solder paste intrusion between adjacent pads.
Stencil support structures are starting to affect the process window. Conventional honeycomb supports have a center deflection exceeding 0.03mm in large-size stencils (≥450×450mm), resulting in the solder paste thickness on middle pins being 11%–15% lower than on edge pins. Currently, two equipment manufacturers have launched air-floating auxiliary support modules, which add a 0.8MPa nitrogen buffer layer on the basis of vacuum adsorption. Measured results show that the deflection is controlled within 0.012mm, but this solution requires the oil content of the compressed air on the production line to be ≤0.01mg/m3; otherwise, gas path blockage will cause support failure and trigger batch solder shortage defects.
Solder Paste Thawing management is seriously underestimated. Most factories still use room-temperature standing thawing, which causes condensed water vapor on the outer wall of the bottle to mix into the paste. We found in a certain medical equipment project that after printing 120 pieces with solder paste thawed without humidity control, the number of Flux phase separation spots visible under a microscope increased by 3.7 times, directly causing the void rate of 0.6mm pin solder joints to soar from 9% to 31%. The correct approach is to first place the refrigerated solder paste in a 25±1℃ constant temperature box with packaging for slow thawing for 2 hours, then unseal and stir for 3 minutes, and the usage time limit after a single bottle is unsealed must not exceed 8 hours.
AOI inspection logic is adapting to through-hole reflow soldering characteristics. Traditional algorithms have a 44% misjudgment rate for identifying solder climb height on pin sidewalls, because the standard library does not include contour distortion caused by post-reflow pin thermal deformation. On-site, a new model has been trained by collecting 500 sets of real solder joint images, changing the sidewall solder climb judgment threshold from a fixed 0.25mm to a dynamic mapping of 0.38–0.45 times the pin diameter, reducing the false positive rate to 6.2%. However, this model still has a 19% missed detection rate for pins with severe plating oxidation, so a plasma cleaning process must be added before printing.
