SMT Preventive Maintenance: Stop 5 Major Problems with ...

Quality nozzles and feeders are the core of pick-and-place. In this article, Zachery Shook, Count On Tools, describes five major issues associated with improper nozzle maintenance and/or the use of poor quality nozzles in the SMT pick-and-place process.

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No SMT equipment can place components accurately and run efficiently without quality nozzles and feeders. These two factors are the core of the pick-and-place process. If the machine is either unable to pick parts consistently or hold on to the components during the transport from feeder to PCB, defects will result. An increase in defects means a decrease in production, costing the company more money over a short period of time. Proper feeder and nozzle maintenance is critical, especially with the current market growth and technological advancements in SMT equipment.

Figure 1. The placement nozzle is responsible for bringing the SMT component from the feeder to the PCB safely, quickly, and accurately.

At first glance, the principle of using vacuum pressure and precision nozzles to enable component placement are basic and straightforward. It is a process that is repeated in every type of SMT assembly equipment. There are five distinct stages to the pick-and-place process:

• Picking: SMT components are withdrawn from a feeder or tray by a vacuum nozzle.
• Holding: components are steadied for rapid movement while the machine detects proper alignment.
• Transport: components are transferred from the picking location to the PCB for assembly.
• Placement: components are lowered to their specific location on the circuit board.
• Release: components are released by the nozzle, which returns to the picking area to restart the process.

Nozzles are the first and last thing to touch all components placed onto a PCB. Pick-and-place nozzles can move tens of thousands of parts every hour (comprising a placement machine&#;s components per hour or CPH rating). With electronic component sizes reaching microscopic proportions, nozzle manufacturers must strive to maintain precision tolerances and exact dimensions in their designs. These nozzles are required to hold the part during transport to the board while the machine is moving and/or rotating at high speeds. The lack of proper nozzle maintenance and/or poor-quality nozzles will lead to part issues, machine issues, and other process problems.

Five Major Maintenance Problems at the Pick-and-place Machine

Each stage of the placement process must be executed repeatedly without failure. Careful review of these processes has determined the following five major issues associated with improper nozzle maintenance and/or the use of poor quality nozzles.

Loss of vacuum. Vacuum loss may be responsible for some issues, as it can prevent the nozzle from picking up a component from the feeder. It also could cause components to shift on the nozzle during transport. One of the main reasons for less suction and vacuum power loss is that nozzle quality is not maintained during the manufacturing process. The quality and structure of the nozzle must match the component(s) it is designed to place. Another issue related to loss of vacuum is poor pick-up location on the component. Poor-quality nozzles can cause extra fatigue on the pick-and-place equipment as it must constantly acclimatize to maintain efficiency.

Short or worn nozzles. Short or worn nozzles result in poor pick up and can cause the part not to be embedded correctly into the solder paste. When the part is not placed into the solder paste correctly, there is not enough surface tension to hold the part while the PCB is moving. Parts will shift on the board. One benefit of monitoring nozzle tip length is that it can allow scheduled preventive maintenance to prevent nozzle tips from causing quality issues.

Nozzle tip wear can also cause less vacuum to be generated, allowing electronic parts to drop or shift during transport. New advancements in ceramics, ESD material, and special coatings allow nozzle manufacturers to design nozzles with exceptional durability and toughness, especially under the extreme conditions in which they work.

Sticking nozzles. When a nozzle sticks, it drastically changes the height at which the nozzle is presented. There are a few things that may cause this to happen. One of the most prominent is the quality of the material used to manufacture the nozzle. Over time, certain plastics and metals can deform, causing the nozzle to not fit correctly in its holder. This results in the nozzle sticking during the picking, transport, or placement stages.

Higher than normal rejection rate at inspection. Causes for higher failures at SMT inspection (visual, AOI, or AXI) include:

• components were not presented to the nozzle in a consistent position;
• poor nozzle lighting from degradation of reflective disks or nozzle face over time, dirty reflective disks or nozzle face, or poor quality of reflective disks or nozzle face;
• nozzle height is incorrect;
• stuck nozzles from part height incorrectly set in program, nozzles worn beyond their tolerances, or incorrect length tolerances on nozzles.

Component and circuit board damage (caused by ESD). With the constant movement of nozzles and components, it is possible to build electrostatic charges (ESD) on the nozzle tips. Once released, this charge can 

Figure 2. ESD during pick-and-place is a major cause of damage.damage vital electronic components being placed. Electrostatic discharge is one of the major causes of device failures in the electronics and semiconductor industry. Manufacturers and users of integrated circuits (IC) must take precautions to avoid ESD during the pick-and-place process. Nozzle manufacturers also must take precautions by researching and developing nozzles that use electrostatic dissipating (also abbreviated ESD) materials.Choosing a Nozzle Supplier

Equipment OEMs are no longer the only option for high-quality SMT nozzles and consumables. With so many nozzle manufacturers out there, it may be difficult to choose the right one. The best process is to factor the price-to-performance ratio. Pick a nozzle manufacturer that offers high-quality products at the most affordable pricing. Request sample nozzles for testing purposes to guarantee that the products perform as well as or better than your current nozzles. Consider the benefits of warranties on parts, and suppliers that serve multiple brands and ages of equipment, if you have a mixed line. If the proper supplier isn&#;t sourced ahead of time, assemblers risk having to put production on hold due to backordered supplies.

Conclusion

Nozzles touch tens of thousands of components every hour. They are highly critical to the pick-and-place process. The need for proper preventive nozzle 

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Figure 3. A tombstoned component.maintenance along with the use of high-quality nozzles is essential.  Properly maintaining pick-and-place equipment, nozzles and feeders can save a company time and money. There are many problems associated with poorly maintaining placement equipment, especially nozzles. Some of these issues include mispicking or misplacing components, tombstoning post-soldering, or flipped components. Other issues include sticking nozzles, component and circuit board damage, and higher rejection rates.

The pick-and-place process is critical to an electronics assembly company. Remember, a down machine that cannot place SMT components cannot make money. With appropriate preventive maintenance measures combined with high-quality nozzles from a reputable source, better results can be achieved on the SMT line.

Zachery Shook, marketing director, Count On Tools/PB Swiss Tools Dist., Hilton Drive, Suite 3, Gainesville, GA ; (770) 538-, ext. 303; Fax: (770) 538-; ; www.cotinc.com.

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Do you have any parts dedicated on the machine? 100n or is probably used on almost everything you build. So why take it off and put it on the machine several times a day. Even if you dedicate only 20 parts to never come off the machine, that should help a bit. Where I was we had over 200 parts that never came off the line. It does slow down the machine a little but done properly it isn't even close to being significant when compared to 4 or 5 changeovers a day on each line.

Another major thing is preloads. As much as possible have the feeders for the next run ready

And don't overlook making sure everything and I mean everything is as close to perfect before SMT even hears about a job.

Some people think the manufacturing process starts at SMT but I say it is the halfway point.

I visited a CM on a one day tour. They were running one job, had the next loaded and were loading the one after. I think it is the only place that looked pre-SMT for issues that everyone else just assumes is SMT. Any complete solution to SMT changeover times will look at pre-SMT as much as they look at SMT.

enough of my rant, remember we are all in this together and keep your stick on the ice.

Steve

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