Circuits Assembly Online Magazine - Overlooked Features of Stencil Printers

Why operators skip important features – and what to do about it.

Automated vision inspection is among the top features of most advanced stencil printers. Other top features and process-related techniques include underside cleaning with vacuum, automatic placement of support pins and auto paste dispensing.

In some PCB assembly camps, vision inspection and other major stencil printer features, as well as setup process techniques, are not used. Reasons for not using them vary. For example, in some instances a stencil printer has vision inspection, but an operator simply ignores it. Other times, it’s not put to use due to poor operator training. A third reason assemblers avoid vision inspection as an option is its high cost.

Several problems can occur if a vision inspection isn’t deployed, especially on highly dense and complex PCBs. Among them: solder bridging, insufficient solder, missing solder, and print misregistration (Figure 1).

Underside cleaning. Underside cleaning is another key feature often overlooked. When a stencil printer machine is programmed to check stencil apertures, it also can be programmed to determine how often it performs that inspection by the number of cycles. Once that program is activated, it looks into the stencil apertures and sees from the initial program that it appears to be a clean aperture, meaning no clogging. But when the camera is activated, it sees stencil clogging. Depending on the percentage of the clogging programmed into the machine, the machine cleans the stencil once it sees the clogging (Figure 2).

An underside cleaning complemented by vacuum, if so equipped, further enhances the cleaning. A vacuum-equipped printer not only unclogs the apertures, but also removes smeared solder paste on the underside of the stencil screen while the automated cleaning operation is underway. Without a vacuum, the particles of some solder pastes that the wiper doesn’t completely clean will move around the apertures to other areas of the stencil underside. And when the next print is performed, the particles will be transferred to the PCB. This is particularly important on PCBs with gold fingers at the edges where solder is not wanted. Hence, those stencils are completely cleaned so that the next print is an extremely fine definition print.

If automatic stencil underside cleaning is not used, the operator is likely to continue printing boards, unaware the apertures are plugged and paste isn’t being deposited. This assembly issue is critical for boards with BGA devices. Once these components are installed, their joints are no longer visible to the human eye. Troubleshooting using x-ray must be used to determine the lack of paste on some locations of a BGA pad. Consequently, that BGA device or devices must be removed for rework. When they’re removed, the device, the PCB and other components on the board are subjected to another heat cycle. Each time boards are reheated, their lifetimes are reduced and reliability problems are induced.

Avoiding or eliminating solder bridges or misalignment also can be programmed into a stencil printer machine. During programming, the machine initially looks at the pads without the paste and then does a comparison after paste deposit. It looks at the differences or grey scale of the pads. The printer then determines if paste deposit is off or right on, and the machine will stop, especially when the more easily detected problem is the solder bridge or excessive solder.

The vision system recognizes the problem; the machine stops, and the operator is alerted. The operator has two options: manual cleaning or a program fine-tuning, doing the necessary adjustment to align the apertures with the pads. This way, boards are built using the right amount of solder, not too much and not too little. When a board lacks the correct amount of solder and is reflowed, it will be too late and costly rework required.

Figure 3 shows the placement of support pins underneath the circuit board is not a feature, but rather a critical aspect of the setup process. Support pins prevent board flexing when the squeegee runs over the top-side. Without the support pins, the result is uneven weight distribution, leaving uneven paste deposits at different PCB locations. This is particularly true when the PCB is populated with fine-pitch QFPs. During printing, the board flexes and the stencil flexes with it, thereby causing solder bridging and misregistration.

Newer machines have automatic solder pin placement, but are rarely used due to inadequate operator training. In some cases, operators resort to manually placing support pins. The problem here is the support pins are often not strategically placed at the proper locations, thus incurring likely printing issues. Figure 3b shows no support pins being used.

Stencil ordering can pose yet another setup process issue with a high probability of adversely affecting delivery time. In this instance, some assemblers order stencils in a foil with no frame as a way to cut costs (Figure 4). The issue here is the inordinate time required to mount the foil into an adapter, especially if an inexperienced operator is involved.

Time-consuming mistakes include mounting the foil upside down or in a reverse manner, as well as damaging it. Additionally, when stencil foils are not stretched properly when mounted on the adapter, it can adversely affect the quality of the paste deposit, causing bridging and misregistration. To avoid this problem, order stencil foils mounted in a frame (Figure 4). (This will incur a moderately higher cost). As a result, there is zero setup time. An operator takes the stencil, ensures it is the right one, inserts it into the machine, uploads the program, and is done.

Not putting into practice. Many assemblers fail to put into practice the available important stencil printer features and tools. Sometimes, due to pressures and demands in shipping schedules, theses features are bypassed, and the front-end process crew is not made aware of it. Then, rework is rushed at the back-end. The product is then subjected to unnecessary heat cycles or rework that can affect long-term reliability, and add delays and labor costs.

Assembly engineers or manufacturing managers can specify the need for a particular feature, fully knowing its strengths and results to reduce the amount of non-value rework into the product. However, in some cases, that feature is overlooked, ignored or improperly used during stencil printing.
Schedule changes may be the cause, triggering this non-use. An operator or supervisor may think that bypassing an already programmed feature can permit a quicker build. If that’s the case, the operator will disable the feature to meet shipment deadlines. More times than not, those PCBs incur failures at the end of the assembly line. Personnel at the beginning of the SMT line aren’t aware of what happens at the end of the line. Rework may be taking place at the end of the line, such as in the testing process, unbeknownst to personnel at the front-end. This is especially true if communication is also a problem on the manufacturing floor.

In the worst cases, by the time front-end SMT line personnel are made aware of the problem, they are already building a different product. Therefore, it becomes difficult to troubleshoot the problem and determine its root cause. They have already built the PCB and are rushing to ship it, but can’t because they are inundated with rework.

Getting full use. Personnel training is the first order of business to fully utilize stencil printer features. Well-organized and disciplined training, mandated from top EMS provider or CM management spearheads efficient use of an advanced stencil printer.

Written procedures or processes documentation augment this training. This document is called “manufacturing process instruction,” and it outlines the program name of a particular board and the stencil printer machine’s setup.

The QC inspector has a checklist of the parameters that are preset on the machine. He or she checks this list against the settings of the machine to make sure all those are turned on. For example, if there is a question or discrepancy there, then the quality person calls it to the attention of the process engineer to resolve the issue. If a deviation emerges, the process engineer has to approve it in writing with a time limit.

The manufacturing process instruction document also clearly spells out features that must be used. For example, procedures covering a stencil printer with a vision system specify the critical PCB locations the vision system is to inspect.

If boards are thin or large, support pin locations must be defined to ensure strategic placement. Those locations must be balanced to minimize board flexing, and if possible, no flexing at all.

The next step is to ensure all these features are used to do a first article of the process. This includes quality control (QC) personnel going through the line, comprehensively reading the documentation, and carefully inspecting the setup of the stencil printer, according to the documentation. All these steps ensure every aspect specified in the documentation is actually being applied. The QC person should not allow the line to run unless all the requirements per the written instructions are met per the formal document.

Once all these procedures are in place and the line is running, the last step is monitoring. Here, the QC technician and SMT line personnel monitor the printer to make sure no errors are being reported. If there are, notify engineering.

If the top features of the machine are fully deployed, personnel do not have to monitor it as frequently as when these features are not used. The stencil printer machine running with its full set of features will be the one to actually provide notification if something goes wrong.

By not fully exploiting these features, operators and SMT line personnel are forced to manually and visually inspect every single board coming out of the stencil printer machine. It is well established that repetitive tasks like this lead to operator fatigue and missed defects. An efficiently programmed stencil printer, complete with its full set of features and setup process techniques, doesn’t overlook these problems. It catches them, alerts the operator, and a problem with the potential of extending engineering costs is quickly resolved.