The formation of solder balls is never caused by a single factor, but is the result of the coupled action of multiple links such as Solder Paste printing offset, unreasonable stencil opening design, abnormal Reflow Soldering temperature profile, and improper matching between PCB pads and solder mask. The most frequently overlooked factor is the suppressing effect on solder paste release capability when the stencil thickness and opening area ratio is less than 0.66, which will force part of the solder paste to be squeezed out from the stencil edge under squeegee pressure and remain outside the pad.

We found that for 0402 package resistors, if the coefficient of variation of printing volume exceeds 12% during SPI inspection, the incidence of solder balls after subsequent reflow will rise to more than 3.7 per thousand pieces. Especially when the solder paste particle size is 25±5μm and the viscosity is 850Pa·s, the high proportion of fine powder will aggravate the collapse tendency of the solder paste. While setting the squeegee angle between 55° and 60° can improve the demolding effect, it is easy to cause secondary solder paste trailing in the QFN large pad area. These trailing materials cannot be fully oxidized under a nitrogen atmosphere and are directly involved in the molten solder to form the core of solder balls.

The influence of PCB solder mask on solder balls is very practical. When the thickness of the green solder mask exceeds 25μm or there is a misalignment of more than 0.05mm with the pad edge, the volatilization of Flux during the reflow stage is blocked, which will push the liquid tin to migrate to the weak part of the solder mask. Especially if the through holes at the bottom of the BGA are not treated with resin plugging, the solder material can easily climb along the hole wall and condense into balls at the top. Such solder balls often have a diameter between 0.1mm and 0.3mm and are firmly attached, making them difficult to be identified by AOI.

The no-clean solder paste used by JiajinYuan has a thixotropic index controlled in the range of 4.2 to 4.8 under an environment of 25℃. If the temperature and humidity fluctuations in the workshop exceed the range of 22±3℃ and 55±10%RH, the apparent viscosity of the solder paste will drop by more than 15%, thereby weakening its anti-collapse ability. At this time, even if the stencil tension is maintained at 38N/cm2, micron-level burrs will still appear on the edge of the solder paste after printing. They will fall off due to mechanical disturbance the moment the vacuum nozzle of the placement machine lifts up and scatter in the surrounding area. These free solder particles are heated and splashed onto the component body or PCB surface at the initial stage of reflow to form isolated solder balls.

To solve the problem of solder balls, we cannot only focus on the reflow oven. We must trace backwards whether the storage conditions of the solder paste meet the refrigeration requirements, whether the stencil cleaning frequency reaches once every 500 prints with ultrasonic cleaning, and whether the wear degree of the placement machine nozzle has exceeded 0.02mm. Because slight deformation of the nozzle inner diameter will change the placement pressure distribution, causing 0201 devices to produce micro-settling of 0.05mm level in the Z-axis direction, thereby squeezing the originally normal solder paste volume and causing local overflow.