How to avoid printing errors through stencil inspection

70 percent of errors in electronics manufacturing occur right at the first process step: printing. This is, at least since SPI systems have conquered the SMD market, a well-known wisdom that gives solder paste inspection its justification. But wouldn't it be better not to produce this scrap in the first place, rather than sorting it out after SPI inspection?

The tricky error rate of 70 percent is not the result of printer manufacturers not having done their homework. It is so high because a large number of parameters - both human and machine-related - come together before the first print, and influence the result. That's why print process optimization starts with the stencil.

Let's take a look at the influence of the stencil on the printing process: How do printing errors occur?

Here, we exclude the manufacturing process of the stencil. But even here, faulty production can occur, which most SMD manufacturers can hardly influence, since stencil production is often commissioned. So we assume here a perfectly produced stencil.

The machine operator clamps this stencil into the printer and production begins. Depending on the printing material (paste or adhesive), a certain number of printing processes are possible until so many residues have collected in the stencil openings that not enough solder is applied and thus a defect is produced. In short: the stencil must be cleaned. Many printing systems already offer exciting tools for cleaning cycles. But how do you check whether the cleaning was sufficiently precise and whether there are still residues in the openings?

In practice, an operator looks over the stencil. But can a human eye really be held responsible for whether a stencil opening of 100µm is clean or not? Keep in mind that a human hair is about 50µm thick. Do your eyes find the hair in the soup, i.e. in the stencil opening?

Because the time required for visual inspection and also the high follow-up costs of a stencil opening that is dirty but not detected by a human being are very significant, it makes sense to automate this process - with a stencil inspection system.

Here, the manufacturer Sunmenta offers a wide range of systems that check the stencil for area, position, offset, size, foreign objects, scratches, missing, clogged or additional openings, and measure stencil tension and thickness.

But does it pay off? Yes, because as explained at the beginning, a critical process step - we remember the 70 percent - becomes much safer, and follow-up costs are averted. In addition, employees are relieved because a stencil inspection system completes the inspection in a much shorter time and at the same time still supplies data for traceability.

Those who shy away from the investment costs and rely on their SPI system, which does check the printing process, should be told: You may find the error, but you haven't done anything to prevent it from happening again. One system inspects the stencil, the other the solder paste. In addition, a stencil inspection system also offers the possibility to check the stencil tension as well as the stencil cut according to specifications upon delivery: Errors in stencil thickness and incorrectly placed openings lead to excessive or insufficient solder paste application as well as offsets - further error parameters that have not even been highlighted in this article.

Because more and smaller components are being packed onto an assembly, the stencil openings must consequently also become smaller and smaller. At the same time, this reduces the scope for error tolerances. The topic of stencil inspection is therefore becoming more important than ever, which is why SmartRep will be presenting its solutions and concepts for this at SMTconnect 2022.

Visit us in hall 4A, Booth 230.