High-Precision Aqueous Cleaning

As more and more functionality is packaged in less and less real estate, aggressive measures must be used to clean circuit pack assemblies. In past years, the cleaning material selection was based primarily on performance and cost-efficiency. However, growing environmental and health concerns have forced users to consider less damaging, more efficient ways to clean populated printed wiring boards.

This article presents the work done by one major network server supplier to introduce semi-aqueous centrifugal cleaning in a low-volume, high-mix surface-mount technology circuit pack production environment. The information presented is a result of one year of continuous use of the cleaning technology in the production of multi-layer double-sided mixed circuit boards.1

The network server's circuit board facility was designed to assemble a broad mix of circuit packs in real-time daily support of the unit assembly shop adjacent to the line. These circuit packs are designed for use in workstations and workgroups as part of a network server system. As with most mix shops, throughput (although a factor) is not the dominate concern of production. Typical machine cycle times range from 2–6 min. To accommodate the variety of product flow, equipment is arranged in "islands of automation," and product movement is accomplished by the equipment operators via a pull system. The main process design goal was to simplify the assembly process while allowing a more operator-directed assembly solution.

Circuit packs are contained in a picture-frame type panel during assembly, provid-ing a common 12 × 18-inch size. One to 20 boards may fit within the panel. The boards are removed from the panel prior to test, usually after cleaning. Typical component mix is 90% surface-mount boards and 10% through-hole boards, although several 100% surface-mount packs are processed. RMA (rosin mild activated) solder paste and foam flux are used for assembly. The circuit board shop produces not only production product but also all of the early pre-production prototypes.

Cleaning is an enabling process step in several electronic assembly production processes. Circuit assembly cleaning removes contaminants generated during the finishing process and aids assembly production by increasing yields. The contaminants generated may result in short term, measurable production losses that are sometimes difficult to quantify. In some cases, contaminants may mechanically impede devices used to test parts for quality. In transactions where one company offers components for final assembly at another company, cleanliness greatly increases the ability to ensure process integrity.

This network server's manufacturer made the decision to clean the boards due to the importance of the products assembled and issues concerning no-clean development affecting product time to market. Product design was under development, and a process was chosen that could handle the majority of the design technology being considered (through-hole, SMT, fine-pitch, etc.). Also, no-clean of SMD (surface-mount device) was just being considered at this facility, and it was felt the development work could hamper the product and process development efforts.

Coupled with the decision to clean, this network server supplier had a mandate to discontinue use of chlorofluorocarbons (CFCs) and other solvents of environmental concern. Equipment was sought that met the aforementioned facility layout and process requirements while having the ability to handle a variety of cleaning methods. The machine had to be simple to operate, easy to startup and shutdown, able to keep pace with assembly and easy to maintain.

BIOACT® EC-7, a terpene solvent manufactured by Petroferm Inc., was chosen as the cleaning agent. Terpene solvent is derived from natural sources and is biodegradable, nontoxic and noncorrosive. In a study by Dickinson, Guth and Wengel2, EC-7 was found to be an excellent cleaning agent of surface-mount assemblies when properly rinsed. The process of first washing then rinsing is termed "semi-aqueous" by the electronics industry. Choosing a terpene cleaner and a semi-aqueous cleaning method allowed the company to continue to use its existing flux and soldering process, assuring the same fundamental soldering quality seen prior to adding the cleaning process.

Terpenes are not new. Various studies have shown them effective biodegradable cleaning solutions. While the cleaning solution has excellent cleaning properties, it also has a low flash point (117F), requiring cleaning manufacturers to face new design requirements. Until new equipment was designed or existing equipment altered, terpene use was a risk in manufacturing.

Speedline Accel developed a method of cleaning that safely used terpenes. The MicroCel was also found to fit perfectly with the customer's manufacturing method. The unit was designed to work with a variety of chemical formulations and process cleaning techniques3.

The machine is a semi-automatic cleaning system that harnesses centrifugal and Coriolis energies by spinning the board on center within an enclosed chamber. Solvent immerses the spinning board during the wash cycle. The solvent is forced in a direction parallel to the plane of the board, driving it under the components. Contaminants are solubilized, suspended in solution and driven off the board. A second solvent rinse flushes away residual contamination and wash solvent. A final drying cycle spins the board while introducing hot air within the enclosed chamber.

A simplified block diagram of the cleaning system is shown in Figure 1. (Table I lists the processing steps) Reusable wash solvent is maintained in a temperature-controlled reservoir. Ten gal of washing solvent is provided from the reservoir to the processing tank prior to the wash cycle. After the washing process, the wash material is returned to the reservoir through a 50-micron filter and re-used. A reservoir coolant system keeps the solvent temperature below a user-determined temperature. Nitrogen is injected into the processing tank to provide an inert atmosphere during the rinse and wash cycles. Tap water is heated and then sprayed into the process chamber to rinse the wash solution. The rinse water is removed to a waste drain and not re-used.

All discharge, including the coolant, is collected in a sump and pumped to a common drain. A permit was granted by the local municipality to discharge to the sanitary sewer. However, the rinse water can be recirculated through carbon particulate filters and a de-ionizing mixed bed. Drying is accomplished by injecting filtered, dry, hot air into the process tank as the board is spinning.

Early lab studies have shown the ability of the cleaning solution to clean surface-mount assemblies using centrifugal methods. These results, plus pre-purchase trials of the equipment, were confirming studies to the process feasibility. The only thing left to determine was actual process parameters and capabilities. These were determined during the acceptance phase after delivery.

The machine operator loads and unloads boards into one of several fixtures that attach to the machine's robot head. Depending on the component height, up to 2 panels can be processed at one time. Once loaded, the operator causes the robot to lower the product into the process chamber via the use of two safely spaced sensor strips. A glass cover hermetically seals the top of the process chamber. Once the chamber is sealed, the machine automatically sequences through the washing process following preprogrammed process settings. The operator is free to work on other tasks until the process is complete. After the cleaning process, the robot raises the product, sounds a signal to indicate process completion and positions the fixture for product removal.

Operating parameters are controlled via an operator console. Three parameters are selectable for each of the three main steps of the cleaning process. Table II lists the main process steps, important step components and a brief description of each. Briefly, these parameters are product rotation speed, number of cycles (directional pairs) for the product and time for one direction of spin. Initial product process parameters can be determined in as little as 15 min using a populated board. Prove-in of a new product, while usually performed by the process engineer, can be done by the process lead person. Machine operation is simple and straightforward. With the glass enclosed cleaning chamber, all processing steps can be viewed and changes made if necessary.

The cleaning system was integrated into the production process. The cleaning solution has been used as the cleaning agent since process introduction. In the 12-month period following introduction more than 40,000 circuit packs were cleaned. Initial and periodic checks for surface contaminants and of surface insulation resistance (SIR) have shown levels well within the manufacturing specifications. Figure 2 shows operating temperatures of the cleaning solution for a given period. This data represent typical operating conditions seen for the cleaning system in production at the facility.

Table III shows the machine setting used for the data in Figure 2. During constant use, the cleaning solution in the reservoir will increase in temperature as a result of the hot air heated process chamber and the running of warm circuit packs. Left unchecked, the temperature will stabilize at around 100F. Using the reservoir cooling system, the temperature is maintained at no more than 85F. These temperatures are a result of washing parameters and machine use. Slower production flow allows the chamber to cool between cycles causing less heating of the wash solvent. Figure 2 depicts almost constant machine use.

Some typical operating parameters associated with cleaning can be seen in Table III. These items, while greatly dependent upon the product being cleaned and the cleaning parameters, will give the reader some idea of typical conditions.

As discussed, a permit to discharge wastewater mixed with cleaner to the municipal sewer was granted. Earlier in production, concerns were expressed about disposal of the cleaner bath. The shop realized the cleaner dragged out during rinsing required replenishing long before the solvent was saturated with flux. In other words, no full-strength cleaner waste was sent to the sanitary sewer. Operators merely replenished the spent fluid via dragout and periodically drained about 1 gal from the lower drain of the reservoir to remove sediment. Since the coolant water drained to the same sump, cleaner concentration was maintained well below safe combustion levels. Figure 3 shows solvent use compared to cards cleaned.

Weekly maintenance consists of adding cleaner to fill the reservoir, checking all seals and cleaning the solvent filter. A spare filter is used to replace the one in the machine. The removed filter is cleaned and then allowed to dry for use again the following week. Monthly, 1 gal of solvent is removed from the reservoir, the rinse drain is checked for blockage and the chamber wipers replaced.

The manufacturer's experience with semi-aqueous centrifugal cleaning of mix-technology boards has been positive. The process developed allowed the greatest manufacturing flexibility while maintaining the quality standards necessary to compete and survive in the next decade.

TABLE I—Centrifugal Semi-Aqueous Cleaning Process
Process Step	Process Component	Process Description
Wash	Close	Lower product into process chamber and seal top.
	Fill	Fill chamber with cleaning solution.
	Wash	Spin product via programmable speed and direction.
	Purge	Drain fluid to reservoir while spinning circuit board to force efficient draining.
Rinse	Rinse	Spin board in different directions at programmable speed while spraying with hot rinse water.
Dry	Dry	Spin board in different directions at programmable speed while injecting dry hot air.
	Open	Raise cover and position fixture for product removal.

TABLE II—Process Parameters
Process Cycle	Speed (rpm)	Cycles	Time (sec)
Wash	225	4	5
Rinse	675	5	4
Dry	600	16	1

TABLE III—Machine Settings for Data in Figure 2
Item Measured	Input Condition	Output Condition
Rinse Water Temperature	135F	127F
Rinse Volume (typical)	NA	0.66 gal/cycle
Drying Air Temperature	325F	Not measured
Reservoir Coolant Temperature	58F	61F
Coolant Volume (per minute)	NA	1.5 gal
Solvent Dragout (per manual avg.)	NA	0.02 gal
Process Capability (12 × 18 inches)	NA	2 panels every 5 min

 

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