If Solder Paste is not stored at a constant temperature of 25±3℃ and is not fully warmed up for more than 4 hours before outbound, it will lead to abnormally increased viscosity of the solder paste and uneven distribution of metal content, which in turn causes poor printing release and excessive solder joint void rate. The root cause is that temperature difference causes Flux precipitation and alloy powder agglomeration. The correct practice is to transfer the refrigerated solder paste to a constant temperature room and let it stand for no less than 4 hours, and use a viscometer to actually measure and confirm that the viscosity has fallen back to the manufacturer's nominal range before use.
If the squeegee angle is fixed at 60° for a long time without dynamic adjustment according to changes in stencil tension, it will cause solder paste to accumulate at the edge of the opening or insufficient filling, especially on 0201 and 0.4mm pitch QFN pads, where the coexistence of insufficient solder and bridging is extremely prone to occur. The root cause is that the tension attenuation degree of different batches of stencils varies greatly. It is necessary to actually measure with an angle meter after each stencil change and perform reverse calibration in combination with the first article SPI data. The initial setting for a new stencil is 55°, and then fine-tune it to between 58° and 62° according to printing stability.
If the solder paste is exposed to the workshop environment for more than 2 hours after opening without being covered with nitrogen protection, its surface will quickly oxidize and absorb moisture in the air, causing spattering and poor wetting during reflow. On-site, it is often misjudged as abnormal furnace temperature, but in fact, it is the dual effect of decreased flux activity and increased solder ball generation. The correct practice is to turn on the nitrogen immediately after opening (flow rate ≥15L/min), and check every 30 minutes whether the sealing ring at the tank mouth is deformed and leaking.
Using Type 4 solder paste (D50=25μm) but matching it with a stencil with an opening aspect ratio <1.2 leads to the solder paste being unable to completely fill the pad opening. After printing, SPI shows that the solder paste volume is 18% to 22% lower. This problem is particularly prominent in double-sided mixed-mount industrial control boards, because the fine-pitch devices on the B side mostly rely on high-precision release. It is necessary to reversely verify the stencil opening aspect ratio according to the solder paste type. The minimum aspect ratio corresponding to Type 4 should be ≥1.5, and that for Type 5 should be ≥1.7.
If the interval between printing completion and placement exceeds 45 minutes without temperature and humidity monitoring, the rosin component in the solder paste will start to crystallize slowly. After placement, it is prone to form a weak wetting interface and discontinuous IMC layer after reflow. X-ray inspection shows grayish-white bands at the edge of solder joints. This phenomenon has a 3-fold increase in incidence in high-temperature and high-humidity summer workshops. The root cause is that rosin recrystallization accelerates when the workshop RH is >60%. It is necessary to install a real-time temperature and humidity sensor at the printing station, set an automatic alarm when RH >55% and pause material supply.
Simple checklist: Confirm that the temperature record on the solder paste batch label is complete, check the calibration validity period of the temperature control probe in the constant temperature room, verify the consistency between the daily stencil tension test value and the squeegee angle setting value, check whether the pressure gauge reading of the nitrogen tank is >0.4MPa, verify whether the solder paste volume alarm threshold of the SPI equipment is updated according to the current solder paste model, and spot-check whether the timestamps of placement after the last 3 printings are all ≤45 minutes.
