When performing pre-reflow inspection on the JJY SMT line, all operations must be completed within the window after placement is finished but before the board enters the reflow oven, as the Solder Paste is still in an unmelted state at this point. Minor misalignment can still be corrected by the placement machine, but once the board passes through the oven, the component can only be scrapped or reworked. Operators must scan station by station along the conveyor direction using a 10x magnifying glass, focusing on comparing whether the solder pad coverage ratio at both ends of CHIP components is consistent. If it is found that the solder paste coverage on one end is less than 60% while the other end exceeds 90%, tombstoning is highly likely to occur during reflow due to unbalanced surface tension. At the same time, synchronously observe whether small-sized resistors and capacitors such as 0201 and 0402 have overall displacement greater than 0.15mm or rotation angle exceeding 3°. This type of misalignment has a high AOI false positive rate, but the accuracy rate of manual re-inspection can reach 98%. Extra caution is especially needed when there is a ±0.05mm design deviation between the stencil aperture and the PCB pad.

When the JJY placement machine performs high-speed placement of 0.4mm pitch QFN, if the nozzle vacuum fluctuation drops below -75kPa, it will cause slight tilting of the IC body. At this time, even if the XY positioning accuracy is within ±0.03mm, the actual contact area between its pins and the solder paste will decrease by more than 20%. This hidden hazard cannot be exposed solely by coordinate data, and must be combined with the side-light oblique illumination method to observe whether there is continuous intermittent reflection at the edge of the solder paste. We once ignored this detail, resulting in unilateral cold solder joints on the QFN of 12 boards in a mass production run, and the rework took longer than redoing the boards. Later, after incorporating side-light inspection into the mandatory first-five-piece process, similar issues were completely eliminated.

For double-sided mixed assembly boards, it is even more necessary to confirm before reflow whether the components placed on the B-side have been scraped and deformed by the A-side stencil wiping mechanism. Especially for connectors or inductors with a height exceeding 1.2mm, their plastic housings will undergo elastic depression at the 0.08mm level after being compressed. Although this deformation does not trigger an AOI size alarm, it will hinder solder paste climbing during reflow and induce local tombstoning. At this time, a thickness gauge needs to be inserted into the gap between the side of the component and the PCB for actual measurement. If the reading is less than the nominal value by 0.1mm, it is judged as a risk item, and the line must be stopped to adjust the wiping pressure parameters or replace the corresponding model of buffer gasket.

If the JJY AOI program does not enable the "solder paste bridging trend analysis" function, it cannot capture in advance the gradual process of minor solder bridging between adjacent pads. This kind of bridging will exacerbate thermal stress concentration during reflow, causing 0402 resistors to flip 90° and tombstone during the cooling phase. Therefore, we require that all new programs must load this module before going online, and set the threshold to trigger re-inspection when the solder paste edge spacing is ≤0.06mm. Combined with cross-validation using manual magnifying glasses, the tombstoning warning time is advanced by an average of 2.3 work order batches.