In the SMT production line of industrial control boards, it is often encountered that the void rate of BGA solder joints exceeds the standard but no Solder Paste batch abnormality can be found, or the same solder paste has a yield of 99.8% on FR-4 boards, but after switching to a 2.0mm thick high-TG industrial control board, the solder bridging rate increases sharply by 12%. Such problems are fundamentally not related to equipment parameters, but the implicit mismatch between the technical indicators of solder paste and the physical characteristics of industrial control boards. For example, the large heat capacity of thick boards leads to a lag in the peak reflow temperature. If the proportion of low-boiling-point solvents in the solder paste is too high, it is easy to volatilize prematurely during the heating stage, causing the Flux film to rupture, which in turn leads to uneven wetting. If the metal content is lower than 88.5%, the solder paste collapse amount in the large copper foil area after printing will be 37% higher than that of conventional consumer PCBs. This is a reproducible physical phenomenon under a microscope, not statistical fluctuation.
Viscosity and thixotropic index must be viewed together, because a solder paste with a nominal viscosity of 650 Pa·s alone may have an actual apparent viscosity of only 210 Pa·s at a shear rate of 100/s. The common squeegee speed for industrial control board SMT is 45 mm/s and the pressure is 0.45 MPa. At this time, solder paste with a thixotropic index lower than 0.55 will have edge drawing on QFN pads with a stencil opening width below 0.25 mm. However, if the thixotropic index exceeds 0.72, it will lead to an excessively high vertical edge of the solder paste after demolding, and self-collapse will occur in the early stage of reflow. We actually tested three mainstream solder pastes in the mass production of a wind power main control board. Among them, type A has a thixotropic index of 0.61, and the standard deviation of height consistency after printing is 6.2 μm; type B has a thixotropic index of 0.53, and the standard deviation reaches 11.8 μm; type C has a thixotropic index of 0.75, although it is clean after demolding, the solder ball occurrence rate after reflow is 2.3 times higher.
There is a strong coupling relationship between metal content and particle size distribution. For solder paste with a nominal metal content of 89.2%, if the D50 particle size is greater than 28 μm, when printing on a 0.3mm pitch ARM core BGA, the friction on the stencil hole wall intensifies, causing the effective transfer rate to drop to 73%. For solder paste with the same 89.2% content but a D50 of 22 μm, the transfer rate can reach 89%. However, the oxide film of the latter is thinner during the reflow stage, and the requirement for nitrogen purity is increased to more than 99.995%. Otherwise, the void rate will soar from 8.7% to 24.1%. This explains why the same factory uses the same reflow oven, and the yield suddenly drops after switching the nitrogen supply source. The problem is not in the furnace temperature curve, but in the rigid dependence of the technical indicators of the solder paste on the atmospheric conditions.
The activity level of the flux cannot be judged without separating the cleaning process. Although the ROL0 level solder paste has a residual insulation resistance of >1×1010Ω, its rosin-based carrier has an obvious carbonization tendency after being kept at 125°C for 180s. A rail transit signal board had excessive leakage current in the high temperature and high humidity aging test. The root cause was that ROL0 solder paste was selected but the board surface ionic contamination (IC) retest was not done. Although ROL1 level solder paste requires water washing, under the conditions of spray pressure of 0.25 MPa and water temperature of 62°C, the IC value is stably controlled at 0.78 μg/cm2 NaCl equivalent, which is far better than the electrochemical migration risk of no-clean type after long-term service.
There should be clear selection directions for different industrial control scenarios. Military industrial control boards must use solder paste with D50≤22 μm, metal content≥89.0%, and thixotropic index 0.58～0.65, and are mandatory to be equipped with nitrogen protection and IC online monitoring. Outdoor industrial control boards such as wind power and rail transit should give priority to ROL1 active solder paste containing halogen inhibitors, because the corrosion rate of its residues in a condensed water environment is 40% lower than that of ROL0. For the production line of PLC basic modules that are extremely cost-sensitive, a balanced solder paste with D50=25～27 μm, metal content 88.5%, and thixotropic index 0.60±0.03 can be accepted, provided that all stencil openings are thickened by 5 μm and the stepped stencil design is cancelled. All selections must take the measured void rate of the factory's own reflow oven <15%, ICT open circuit rate <30ppm, and intact function after HALT as hard thresholds. If they are not met, switch immediately without exception.
