After Solder Paste printing, obvious burrs along the edges and a large amount of granular residue are observed under a microscope. The root cause is that the rosin carrier in the solder paste has excessively high viscosity at room temperature and a low thixotropic index, leading to stringing and fracture during squeegee peeling. The practical solution is to let the solder paste stand for 4 hours after being taken out from the constant-temperature 25°C storage before machine operation, while fine-tuning the squeegee angle from 60° to 58° and reducing the printing pressure by 0.2 kgf. The key preventive measure is to use a viscometer to measure the apparent viscosity of the solder paste before each material change to ensure it falls within the manufacturer's nominal value ±15%, while recording ambient temperature and humidity and calibrating the stencil tension gauge synchronously.
After Reflow Soldering, the void rate at the bottom of BGA components detected by X-ray generally exceeds 25%. The symptom is that voids in a single component are concentrated in the central area with irregular shapes. The main cause is that the lead-free solder paste used has a metal content of only 88.5% and a relatively high Flux proportion, which blocks the gas escape channel during the high-temperature phase. The practical solution is to switch to a solder paste model with 90.2% metal content, lower the peak temperature from 245°C to 242°C, and extend the holding time by 12 seconds. The key preventive measure is to request the original SGS composition report for each batch of solder paste upon warehousing, focusing on verifying whether the Sn/Ag/Cu ratio is 96.5/3.0/0.5 and ensuring no low-melting-point doping elements such as Bi or In are present.
When manually soldering high-voltage drive modules, frequent issues of blackened solder joints and poor wetting occur. The phenomenon is that after the soldering iron is removed, the solder joints appear grayish-black and matte, with edges shrinking into a spherical shape. The cause is that the purchased Solder Wire has a nominal silver content of 2%, but the actual measurement is only 1.3%, leading to an increased liquidus temperature and making it difficult for the flux to effectively remove the oxide film. The practical solution is to immediately stop using this batch of solder wire, switch to solder wire with a silver content of 2.0±0.1% confirmed by EDS in the FA laboratory, reduce the soldering iron temperature from 340°C to 325°C, and strictly control the contact time within 3 seconds. The key preventive measure is that all solder wires must be sampled and sent to a third party for ICP-OES full element analysis after arrival, and they cannot be put into production before the results are issued.
After reflow of the vehicle OBC circuit board, a large number of tombstoning phenomena are found, especially for 0402 packaged MLCCs, which account for 18%. The symptom is that one end of the component stands upright while the other end adheres to the pad. The cause is that the activator in the solder paste volatilizes too quickly in the preheating zone, leading to an imbalance in surface tension between the solder joints at both ends. The practical solution is to slow down the heating slope of zone 2 (150°C section) from 1.8°C/s to 1.2°C/s, and use trapezoidal openings instead of square ones for small-sized components in the stencil aperture design. The key preventive measure is that before introducing a new solder paste, tombstoning rate comparison tests under three sets of different temperature zone slopes must be completed, and the mass production process can only be switched after the data meets the standards.
Within 24 hours after the solder paste is opened, the printing yield drops sharply by 12%. The macroscopic manifestations are poor release and insufficient solder, and a semi-transparent film-like substance is visible on the surface of the solder paste under a microscope. The cause is that the rosin system of this solder paste is sensitive to moisture in the air, and local crosslinking easily occurs when RH > 60%. The practical solution is to activate the nitrogen-protected printing machine and install a silicone sealing ring at the opening of the solder paste can, while strictly controlling the workshop humidity from 65% to 45±3%. The key preventive measure is that all solder paste cans must be marked with the opening time and responsible person, and any solder paste that has not been used up after more than 8 hours must be scrapped and cannot be re-warmed for repeated use.
In the same furnace batch, the thickness of the IMC layer at the solder joints of the IGBT drive chip is uneven. FIB cross-section shows that the Cu6Sn5 layer at some solder joints is as thick as 3.2 μm, while adjacent solder joints are only 1.1 μm. The cause is that the Sn particle size distribution in the solder paste is broad, with the maximum particle size exceeding 25 μm, leading to large differences in thermal response time between solder joints during melting. The practical solution is to replace it with a narrow-distribution solder paste with D50 = 18±2 μm, and add a 10-second nitrogen pre-purge before reflow to remove adsorbed water on the pad surface. The key preventive measure is that each batch of solder paste must provide the original laser particle size analysis spectrum, and the D90 value must not exceed 22 μm, otherwise it will be rejected.
