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Monitoring Precision Parts Cleaning

AbstractContinued miniaturization of high technology products in a wide range of industries has resulted in the increased need for cleaner parts. Continuous particle monitoring utilizing a wide range of sampling techniques is capable of meeting the challenges and complexity of today’s precision cleaning systems. Particle monitoring of cleaning systems has been shown to increase product yield and overall productivity. In addition, monitoring at multiple points in a cleaning system speeds identification of root cause for out-of-specification situations. Finally, statistical analysis helps establish the optimum operating parameters resulting in the highest throughput and lowest failure rate.

IntroductionContamination on the surfaces of electronic parts and mechanical assemblies can degrade performance, production yield and life expectancy of a product. Manufacturers have instituted cleaning procedures to ensure their precision component parts are clean and unlikely to adversely effect the completed product.

In addition, increasing regulations due to the environmental impact have greatly curtailed the use of many cleaning solvents, increasing the manufacturer’s reliance on water or water plus surfactant based cleaning processes. While many different types of contamination are removed through these cleaning processes, particulate contamination often remains critical. However, this is easily monitored with the current technology of particle counters. Continuously monitoring particle concentration in a precision parts cleaner provides a real-time indication of the amount of contamination remaining on a part.

Rigibot™ Automated Cantilever Transport System

Custom Fixtured Baskets

Operator Controls

Sliding Dryer Cover

Power Box

Dryer Filter Chamber

Tanks

Load Station

Genesis™ GeneratorsUltrasonic Wash

Heated Ultrasonic Rinse

High Efficiency Recirculating Hot Air Dryer

Figure 1. A typical precision parts cleaner

Monitoring Precision Parts Cleaning

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Precision Parts CleanerPrecision cleaning systems like the example shown in Figure 1 are used extensively in the disk drive industry, where a wide range of assemblies and mechanical parts of various shapes and sizes are used in their products. In this particular cleaner, parts undergo a multi-step cleaning process. Parts are initially placed in a surfactant bath and subjected to ultrasonic waves, which form cavitation bubbles. The formation and collapse of the cavitation bubbles literally scrub the many faceted surfaces of the mechanical parts. After the surfactant bath, the parts are sprayed with clean DI water and placed in progressively cleaner DI rinse baths. The DI rinse baths subject the parts to more ultrasonic waves and a continuous flow of clean DI water. After the last DI rinse bath, the parts are dried and taken to the production line.

Cleaning System ChallengesMost precision cleaning systems rely on ultrasonics, but the individual baths can be solvent, chemical or aqueous based. All three are often relied on, with the solvent baths preceding the aqueous baths. The aqueous baths are either water plus surfactant based or simply ultra pure DI water. These baths may be either continually recirculating or non-recirculating. Table 1 shows a list of select industries and their application needs for product cleanliness, as well the typical particle size of concern. The complexity of these cleaning systems leads to a variety of potential problems including:

• Filter failure and loading

• Fluidics degradation

• Water supply issues

• Unusually dirty incoming parts or parts of different contamination levels

• Variable lot sizes, parts sizes, loads, and part surface areas

• Load spacing and upper and lower control limits for cleanliness not optimized based on performance data

Furthermore, the complexity of these challenges means that one size does not fit all. There are a variety of potential solutions including the number and type of particle counters employed. Multiple particle counters used at different locations in the cleaning system can help identify the cause and speed the resolution of an out-of-spec situation more quickly.

Table 1. Application requirements for parts cleaning within select industries

INDUSTRY APPLICATION PARTICLE SIZE OF CONCERN

Micro Electronics Tool ComponentsWafer Handling and StorageMEMS Sensors

< 100 nm< 100 nm< 100 nm

Data Storage Tool ComponentsDrive Components

100 nm - 500 nm100 nm - 500 nm

Aerospace SensorsFuel ComponentsSpecialty Parts

>2 microns>2 microns>2 microns

Automotive SensorsFuel Injectors

>2 microns>2 microns

Medical Devices ImplantsProduction Equipment

>10 microns>10 microns

Monitoring Precision Parts Cleaning

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Particle Counter SolutionsThe critical variable in the cleaning process is whether the parts are clean enough to use in product or not. The out-going part cleanliness is most often determined using a particle counter to monitor the final DI rinse bath. Particle counters quantify the number of particles still being removed from part surfaces and therefore provide an indication of the contamination remaining on the parts before they reach downstream manufacturing process. There are a variety of particle counter based solutions and these can be integrated into the parts cleaner in several different ways. Figure 2 shows direct sampling from the cleaning bath for syringe samplers, compression samplers and pump samplers. It also shows a pressurized line location suitable for an inline particle counter.

If the rinse bath is recirculated or if the bath provides an isolated overflow return with sufficient head height, a low cost solution would be a continuous inline particle counter, such as the LiQuilaz®, shown in Figure 3. Multiple particle counters at different locations may be used to match particle sizing requirements of the process being monitored.

Syringe based systems (like the SLS-1100 shown in Figure 4) are popular due to their small sample volume and portability. The SLS system achieves high precision through accurate control of a syringe and simple flow-path for delivering liquid samples from a sample vessel or cleaning bath directly to the particle sensor. Its portability makes it an ideal tool for troubleshooting multiple locations.

The approach illustrated in Figure 5 shows a PMS CLS-700 compression sampler. The CLS-700 draws rinse water into the sampler under vacuum. Once proper fluid levels are reached the inlet valve closes and the chemical is pressurized to eliminate bubbles. After a user specified pressurization delay, the fluid flows through the particle counter at a constant pressure and flow rate. This pressurization also makes the CLS-700 useful for monitoring upstream of solvent or chemical baths, where the formation of bubbles are typically more of a problem. Non-recirculated DI rinse systems normally use CLS-700 or SLS samplers for their ability to deliver fluid to the particle counter without any major plumbing modifications.

to inline particle counter

pump

sample location for:• syringe sampler• compression sampler• pump sampler

return

filter

Figure 2. Representation of Particle Counter Sampling Locations from Cleaning System

Figure 4. Syringe Sampler SLS-1100

Figure 3. LiQuilaz Inline Particle Counter

Monitoring Precision Parts Cleaning

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Finally, pump samplers (see Figure 6) are also used in the same fashion as syringe systems and have similar benefits. In this case, a continuous pump is used to move the sample instead of a syringe or vacuum. Pump samplers essentially create a continuous fluid flow and can support faster data collection than syringe or compression type samplers.

Minimum Particle Sizing RequirementsSyringe, compression and pump samplers all include the LiQuilaz inline particle counters. These particle counters come in a wide range of sensitivity limits, from 200 nanometers to 25000 nanometers (25 microns).

For measurement requirements below 200 nanometers, particle counter solutions are available at 100, 65, 50 and 40 nanometers. Syringe based solutions at 100 and 40 nanometers use the UltraChem® generation of products. The UltraChem products are based on NanoVision Technology®. For more information on the potential advantages of NanoVision in chemical process monitoring, please refer to Particle Measuring Systems’ application note, NanoVision Technology Enables Particle Monitoring of High Molecular Scatter Chemistries.

Monitoring Incoming Water Quality All precision aqueous cleaners rely on clean DI water to act as a rinse agent. This clean DI water is normally provided by the site’s DI water filtration system. The precision cleaner is almost entirely dependent on the cleanliness of the incoming DI water which is a good reason to also monitor the final quality of the DI water, the final quality of which should be monitored before distribution at a bare minimum. If the incoming DI rinse water becomes contaminated, its ability to clean parts is degraded.

Figure 7. DI Water Plant Filtration Failure

Figure 5. Compression sampler CLS-700

Figure 6. Pump Sampler

Monitoring Precision Parts Cleaning

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A DI water system failure can be gradual or abrupt. Figure 7 shows the final quality of DI prior to distribution. This water system had both an abrupt and gradual failure within a one month period. The abrupt failure on February 13th was a major leak caused by a seal failure in the water system. Once fixed the water system returned to normal operation. The gradual failure of the resin beds started around February 15th and continued until the resin material was replaced on February 23rd. During the time of the resin failure, particle levels in the DI distribution system were 2 to10 times the typical background level. This water was flowing directly into the parts cleaning system.

For additional information on modern DI water plant monitoring, please refer to Particle Measuring Systems’ application note, Monitoring of UPW Systems Using Ultra DI® 20.

Parts Cleaning SignaturesIn a normally functioning parts cleaning system, the particle concentration in the final rinse bath is normally very low and consistent when product is not being run. When product enters the final rinse bath there is a corresponding increase in particle concentration. The magnitude of the initial particle count spike is an indication of the contamination on parts before processing in the final rinse bath. The particle levels measured when the parts are removed is an indication of contamination still remaining on the parts following the final rinse. Figure 8 shows a typical baseline in the rinse bath and the particle signature generated by parts cleaning.

Evaluating the Cleaning RecipeMany companies attempt to clean parts as fast as their system can handle them. The throughput of a cleaner is often determined solely by the speed of the robotic delivery system, or on general guidelines given by the cleaner manufacturer. However, only a few precision cleaner manufacturers have the time, equipment and resources to determine optimal cleaning parameters for all the parts a given manufacturer may use. Ultimately, the tool owner must determine what processing times, temperatures and settings produce the cleanest parts. Figure 9 shows what happens when parts are processed faster than the rinse bath can recover. Rather than the return to baseline seen in Figure 8, the rapid processing produces a stair-step signature with a gradually increasing particle concentration. The stair-step signature should alert the tool user to modify their processing recipe.

Figure 8. Normal Processing and Return to Baseline Levels in a Rinse Tank

Monitoring Precision Parts Cleaning

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As illustrated in Figure 9, following each part lot processed, there is a consistent bath recovery rate. The time necessary to return the bath to normal baseline levels (recovery time) is a function of the flow rate of fresh DI water entering the bath, the particle removal efficiency of the recirculated system and the level of contamination initially present. Using the observed recovery rate, an operator can determine the optimal processing frequency for a particular tool or type of part. Once an operator determines appropriate processing frequency, they can use the particle signature to optimize the cleaning process. Adjusting one or more of the following cleaning parameters can help tune the system to its best performance and highest throughput:

• Flow of DI water through rinse baths

• Ultrasonic frequency, intensity and duration

• Type and concentration of surfactant

• Temperature of wash and rinse baths

• Duration of wash and rinse processes

The time necessary to return the bath to normal baseline levels will be affected by any changes in lot sizes, part sizes, loads, and part surface areas. Therefore, cleanup time, and both upper and lower control limits must be determined for each type of part and lot size being cleaned.

Cleaning System WatchdogIdeally, a particle monitoring system will provide a go/no-go flag for the tool operator, indicating that the tool is ready to run additional product and whether the last part cleaned is suitable for production. To accomplish this, the tool user must evaluate the characteristics of their system over a period of time. Specifically, enough data must be gathered to develop a clear value for the normal operational baseline and part cleaning signature of the cleaning system (Figure 8).

Figure 9. Effects of cleaning multiple part lots in too rapid of succession

Monitoring Precision Parts Cleaning

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A simple way to determine a baseline (lower) control limit, is by calculating the average and standard deviation of the particle concentration excluding periods when parts are being cleaned. An analysis of the data presented in Figure 8 results in the following statistics:

Average: 22.86 particles per ml Standard Deviation: 4.34 particles per ml

To set a lower control limit at better than 95% confidence, add 2 times the standard deviation to the average. Using this value (31.54) as an alarm limit, the particle monitoring system can indicate if the rinse bath is ready to process parts again.

It is often difficult to quantify if a high particle spike during parts cleaning will result in a failed product, but it is straightforward to determine if the particle spike is within normal ranges. To determine the upper control limit for part cleanliness, the same type of study described in the previous paragraph can be used. In this case, use the maximum particle concentration seen over many parts cleaning runs to determine the average and standard deviation. Using 20 parts cleaning runs (of similar part types and quantities), the following data was calculated:

Average: 2265.1 particles per ml Standard Deviation: 221.9 particles per ml

To set a control limit at better than 95% confidence that a part is dirtier than normal, add 2 times the standard deviation to the average. Using this value (2708.9) as an upper alarm limit, the particle monitoring system can indicate if the part is ready for production or if it requires further cleaning. Figure 10 illustrates when the particle monitoring system would generate a color-coded alarm notification. In this case, the sensor status indication is green when the rinse bath is ready for product to run, and red if parts are too dirty.

There are more advanced means of determining part cleanliness such as:

• Total particles counted while parts are in the bath

• Final particle levels when parts are removed from the bath

However, implementing these requires two way communications between the particle counter system and the cleaning tool control system.

Figure 10. Parts Cleaning with Control Limits in Place

Monitoring Precision Parts Cleaning

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Integrated Inspection StationsThe PMS Surfex® is a final inspection station with precision control of all parameters involved in the ultrasonic extraction and measurement of particles. This greatly facilitates development of optimized cleaning process recipes and can be used as a controlled and repeatable IQA/OQA part cleanliness analysis tool. The Surfex can be integrated with any of the LiQiuilaz or UltraChem particle counters. The Surfex is a final inspection station with precision control of all parameters involved in particle extraction and measurements. Surfex compliments the continuous process monitoring presented in this paper with recipe-controlled, offline cleanliness verification using the same particle counters for consistency. For more information on this integrated approach see the Particle Measuring Systems website.

SummaryContinuously measuring particle concentration in a precision parts cleaner can identify problems caused by internal tool or external system failures. Monitoring at multiple locations provides a rapid identification of root cause saving both time and money. Determination of average particle concentrations and clean-up rates allow operators to adjust process parameters and tune a tool for maximum performance and throughput.

Particle counters are available with sensitivity limits down to 40 nanometers to meet the sizing requirements of any cleaning process. Multiple sample introduction systems, each with their own set of benefits, are available to match customer needs.

Once the tool is operating at peak efficiency, the particle monitor provides statistical process control information which ensures the cleanliness of all parts. Exceedingly dirty parts can be identified and re-cleaned before they reach production, reducing the potential for dirty parts to impact product yields.

Figure 11. PMS Surfex 200 Inspection Station

© 2016 Particle Measuring Systems. All rights reserved. LiQuilaz®, UltraChem®, Surfex®, Ultra DI® 20 and NanoVision Technology® are registered trademarks of Particle Measuring Systems. Reproduction or translation of any part of this work without the permission of the copyright owner is unlawful. Requests for permission or further information should be addressed to Particle Measuring Systems, Inc. at 1-800-238-1801 App Note 33 Rev B 03/2016

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