This article explains that smaller components and narrower pitches present new challenges for board assembly. Understanding these placement issues can lead to faster time-to-market and fewer defects.

The reality facing the industry is that parts are getting smaller and smaller. For example, the 0201 chip capacitor is 75% smaller than the 0402 and occupies 66% less area on the circuit board. These components will appear on some common printed circuit boards early this decade, and the even smaller 01005 chip Shaped components will be seen on modular circuit boards where space is more at a premium by 2005.

Because board space is so precious for many new products, smaller components will be used even more widely despite their higher cost. This new miniaturization requires increased placement accuracy without sacrificing speed.

Identify the challenges

Small components present many problems. Higher densities – the main reason that plagues smaller components – make placement tasks an order of magnitude more difficult. For example, 0201 components usually require a smaller pad size to prevent solder smudges and allow fillet-free soldering. Also, smaller pads mean narrower component pitches. While these allow designers to achieve the higher densities required for highly functional and compact products, they also complicate the situation. For high-density PCBs, placement accuracy directly affects the number of assembly defects after reflow soldering, for example, placement skew increases the chances of solder bridges, solder balls, component erection, and component misalignment to pads.

So what do we need? Now, the realistic production goal is to achieve 99.9% pick-up rate, while the placement accuracy of 3σ is ±60µm. To achieve this goal, machine precision becomes a primary concern.

For example, Motorola’s tests have shown that variations as small as 0.025mm in placement skew can significantly affect defect levels. For standard pads (tested with 100,000 components), placement offsets of y<0.075mm, x<0.075mm have a similar effect on defects as no offset. However, when the offset increased to <0.1mm, the defect level rose to over 5000ppm. While this absolute distance is meant to be small, studies have shown that the process leaves little room for error. Also, the placement operation involves more than itself. It includes pick reliability, accurate component visual identification and placement repeatability. In fact, tests have shown that 0201 components require 99% pick-up reliability.

In order to maintain the coherence of the production system, the nozzle must be able to move in all three directions, i.e. along the X, Y and Z axes – this is important because on all production machines there is no control of the Y axis. However, to keep placement accuracy within tolerances, Y-direction control is necessary to center the component on the nozzle. Naturally, this alignment has tighter tolerances for 0201 than for other parts. The Y direction is the single most important axial correction for 0201 component placement. Due to closed-loop real-time feedback in all three axes, the need for feeder calibration is virtually eliminated. Without real-time closed-loop feedback on the three axes, calibration of the feeder is critical.

Studies have shown that an accuracy of ±0.07mm in the Y direction is necessary to ensure successful 0201 placement. Also, successful placement requires a tolerance of ±0.1mm in the X direction and ±0.1mm in the Z direction to achieve the target value of 0.2mm. Correcting the movement of the nozzle X/Y axis is the key to ensure stable and continuous component picking.

movement on solder paste

Another placement problem is that under certain conditions, the 0201 will not stay where it was placed. Consider such a situation, try to mount the 0201 capacitor on the PCB printed with solder paste and flux, hoping to get a controlled stroke of ±0.05mm and a component spacing of 0.15mm. Experiments have shown that for a placement accuracy of 3σ in the Y direction, components with less than 0.05mm overtravel on the board will sometimes slide more than 60µm toward the short side.

what will happen Interesting further investigation revealed that when the component is simply mounted on flux, the component does not slip due to overtravel

, but occurs when on solder paste. Conclusion: The problem is the particle diameter of the solder paste. To compensate for Z-axis corrections, the machine must have a real-time feedback mechanism that measures the thickness of each component.

When the particle size is larger than 20µm, there is a possibility of component skew because the particles are not evenly distributed on the pad. Because component placement times are milliseconds, any unevenness in the surface can cause part deflection or movement. This is why hot-air solder-applied (HASL) boards are not suitable for 0201 mounting, in contrast to 0402 licensed HASL.

When the component overtravel impacts the solder particles, the reaction force changes the axial direction of the suction nozzle and generates a horizontal force, resulting in the deviation of the component

Therefore, overtravel reduces placement accuracy. It may also increase solder bridging in high-density mounts, as the component pushes solder paste out from under the part when using filletless pads. Therefore, overtravel can be defined as such that the gap between the component and the PCB is smaller than the solder grain size, ie the placement system must control this gap, keeping it at 40-60µm. A contributing factor is the support of the board, without support elements could fall from excessive heights or be pressed into the solder paste. To accurately control travel, the board’s support system must provide sufficient correction for the board’s arch.

improvement needed

To achieve effective use of 0201 components, part of the solution will be found in nozzle design improvements. Because the components are so small, they require a nozzle design that maximizes the vacuum’s contact surface area while providing a form factor that does not interfere with high-density layouts. In addition, the suction nozzle must be highly wear-resistant, since its corrosive effect is exacerbated by the small contact area. All of these must be aware of how to meet and deal with the upcoming challenges of 01005 components.

Current results provide a cycle time of 0.75 for a gap of 0.25mm, an accuracy of 60µm (3σ), and a pick-up rate of 99.9%. The goal was to achieve a takt time of 0.075 seconds per part, an accuracy of 40µm (3σ), and a pick-up rate of 99.9% for a gap of 0.10-0.15mm. A specially developed tape feeder for 0201 components should also facilitate more accurate and faster component placement.

in conclusion

A real economic concern is that boards produced with 0201 components will be more expensive than their larger counterparts. Additionally, tighter tolerances necessitate increased process control, more thorough preventive maintenance, more training and process knowledge, and increased awareness of scrapping and inspection/repair activities.

Preventive maintenance was always an important part of production, now even more so thanks to 0201 placement. Because the tolerances and accessibility of errors are as small as the components themselves, preventive maintenance is a more important manufacturing cost factor for 0201 lines than for other components. Similarly, it appears that the use of 0201 will require more frequent nozzle cleaning, camera cleaning and machine placement measurements and adjustments. The height of component pickup and placement will be critical. For initial component pickup, feeder axis adjustments will be required, although the machine can compensate for component misalignment at the pickup position.

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