During daily machine adjustment on the production line, we found that when the Solder Paste printing thickness deviation exceeds ±15%, the stencil aperture size does not match the pad, or the PCB support platform has slight deformation, even though the visual recognition success rate of Jia Jinyuan placement machines is as high as 99.98%, the solder paste deformation will still cause the component offset after placement to exceed the 0.05mm tolerance band, which in turn leads to defects such as tombstoning, solder bridging, or cold solder joints.

Before placement, it must be confirmed that the solder paste has been warmed up for no less than 4 hours and stirred evenly; otherwise, fluctuations in Solder Paste Viscosity will directly cause edge collapse or surface stringing after stencil printing. At this time, even if the XYθ compensation parameters of the placement machine are all calibrated properly, there will still be insufficient contact area between IC pins and solder paste, which is particularly obvious on QFP devices with 0.4mm pitch.

The nozzle vacuum value of Jia Jinyuan placement machines must be stable within the range of -85kPa±3kPa. If the vacuum fluctuation exceeds ±8kPa, missing pickup or double pickup will occur when picking up 0402 resistors. Such abnormalities are often related to excessive moisture content in compressed air, clogged filters, or slight leakage in gas path joints. At this time, even if the solder paste is in good condition and the pad design is compliant, the final placement position will still have systematic offset.

We once tracked the placement defect rate of a certain batch of BGA devices for three consecutive days, and finally identified the root cause as repeated fluctuations in Solder Paste Storage temperature between 2℃ and 8℃, which reduced the activity of the Flux. After placement, the flux failed to effectively wet the solder ball surface, causing the solder joint void rate to increase by 37% during the reflow stage. This problem was fully reproduced when using the same Jia Jinyuan equipment, the same program, and the same Feeder status.

The placement pressure setting must be adjusted in conjunction with the component body height, the flatness of the tray carrier, and the thixotropy of the solder paste. For example, for a 01005 capacitor with a height of only 0.35mm, if the default 0.15N pressure is used, it will easily press too deep into the solder paste layer, causing the surrounding solder paste to be squeezed out, which instead reduces the wetting effect of the solder end. At this time, the pressure should be synchronously reduced to 0.08N and the Z-axis downward pressing time during placement should be shortened to within 120ms.

Every time a Feeder of a different package is replaced, the offset between the nozzle center and the placement coordinates must be re-verified. Especially when using a tape-and-reel Feeder to transport QFN-type devices, if the X-direction cumulative error caused by changes in tape tension is not corrected in time, it may cause the lateral offset of an entire row of devices on the solder paste-printed pads to reach more than 0.06mm. Such deviations are often misjudged as solder paste printing offset rather than placement problems during AOI inspection.

The Mark point recognition logic of Jia Jinyuan equipment relies on contrast threshold settings. When the surface reflection after solder paste printing is uneven, or the PCB pads are slightly oxidized but not yet at the scrap standard, the vision system may mistakenly identify the solder paste edge as the Mark boundary, thereby causing the entire board coordinate system to deflect. At this time, the image gain parameters must be manually adjusted, and it must be synchronously checked whether solder paste batches from different suppliers and models are mixed.