Once the soldering activity of the Solder Paste exceeds the process window range, it will decompose violently during the reflow process and release a large amount of acidic gas. These gases will form tiny bubbles that are difficult to escape at the bottom of the BGA solder balls, which will lead to an excessive void rate of solder joints. We have encountered a case where the void area accounted for more than 25% in the mass production of a certain customer, which directly caused the thermal resistance of the chip to increase and early failure; at the same time, the halide ions in the high-activity solder paste residue will undergo electrochemical migration after absorbing moisture on the PCB surface, especially when the distance between the two ends of the 0402 resistor is less than 0.3mm, it is extremely easy to induce an increase in leakage current and even a short circuit.

If the activity of the solder paste is too high in the printing process, its rosin matrix is more likely to have a sudden drop in viscosity in the stencil opening. A slight fluctuation in the squeegee pressure will lead to solder paste collapse or edge trailing. This phenomenon is particularly obvious when the ambient temperature exceeds 28°C and the relative humidity is higher than 65%. We have therefore failed the SPI test results of the same WiFi module three times in a row; what is more troublesome is that excessive activation of the solder paste in the reflow stage will weaken the wetting and spreading ability of the solder to the PAD, which will instead cause cold solder joints or head-in-pillow defects, especially in the central area of the bottom pad of the QFN, discontinuous metal connection bands often appear.

If the cleaning process fails to completely remove the high-activity solder paste residue, its chloride component will continue to slowly precipitate after the subsequent conformal coating is applied, which will not only cause local blistering and peeling of the paint film, but also form green copper salt crystals at the contact points of the ICT test probes. A certain automotive electronics customer therefore received a batch return from the terminal factory after batch shipment; in addition, excessively high solder paste activity will also accelerate the oxidation and discoloration of the solder paste on the stencil, changing from the original light yellow to dark brown. At this time, even if the SPI determines that the thickness is qualified, the actual Solder Joint Strength will also decrease by about 18%. We used a thrust tester to measure and found that the average thrust value of 0603 components dropped from 2.3N to 1.9N.

During on-site debugging, it is recommended to first reduce the reflow peak temperature by 2 to 3°C to suppress the excessive reaction of the solder paste, and at the same time shorten the soaking zone time by 15 seconds to reduce the total amount of acidic substances decomposed. However, it is necessary to simultaneously confirm whether the Tg parameters provided by the Solder Paste Manufacturer support this adjustment, otherwise it may cause the problem of incomplete melting of the solder; for the high-activity models that have been put into production, be sure to increase the frequency of wiping the bottom of the stencil once every two hours, and use an anhydrous solvent with an isopropanol concentration of not less than 98.5% to avoid aggravating the corrosion risk due to the water content of the solvent.