In the past, “low residue - no clean” processing has played a dominant role in electronics manufacturing, and the cleanliness of final assemblies was no longer a critical issue in the manufacturing process. Recently, however, with the advent of ultra fast processing speeds, smaller components, denser assembly packaging and the use of more aggressive flux chemistries on alternate plating finishes, the cleanliness of final products is once again a dominant issue.

The ability to effectively and reliably clean under component packages can greatly affect the manufacturing yield by improving electrical contact during the test, preventing corrosion from residual flux contamination and also enhancing the appearance of the final assembly. Today, an ever increasing number of manufacturers are realizing the necessity of final product cleanliness and are seeking effective solutions to meet that need.

Traditionally, manufacturing companies have chosen to use a rosin mildly active (RMA) flux because of the solder reliability. The RMA flux is very aggressive and very forgiving in all processes while maintaining excellent solderability. While the RMA flux provides a good solder joint, it must also be completely removed to prevent deterioration of the circuitry. The flux requires chemistry or a cleaning agent to remove the contamination, as these fluxes are not soluble in water. In the past, manufacturers used vapor degreasers containing hazardous chemicals.

Today, flux contamination can be removed with EPA-approved, water-based chemistries. To date, many military based manufacturers still specify an RMA flux, as it is a proven technology with long-term reliability. In the late 1980’s and early 1990’s, the industry pushed to move towards Organically Activated (OA)/water-soluble fluxes. Although these fluxes are milder than RMA’s, they are still aggressive, still corrosive, and still reliable, providing a quality solder joint. These fluxes also required the manufacturer to remove the flux contamination from the PCB, which could only be achieved using water (de-ionized (DI) preferred). Both water-soluble and RMA contamination can cause test and field failures. PCB manufacturers feel that if they clean either with water only or with water containing a water-based chemistry, they will produce a more stable product. No-clean fluxes are much milder than water soluble fluxes. This means that the parts to be soldered (boards and components) must be more solderable for a no-clean process than a water soluble process. If they are not, an increase in post-solder touch-up may be required. As a result of limitations in the solderability of no-clean fluxes, the window of the soldering process is greatly reduced. Many PCB manufacturers will clean even though they are running a no-clean process. The removal of solder balls and the removal of any remaining residue are two of the many reasons for the continued cleaning.

There are PCB manufacturers who originally switched to a no-clean process and have gone back to a water-soluble process. They have found that the unforeseen difficulties they encountered outweighed the originally forecasted savings of switching to the no-clean process. After cleaning, the PCB assembly will show significantly lower levels of ionic contamination as compared to PCB assemblies that are not cleaned. The EMPF uses a Technical Devices Company Aqueous Cleaner, Model 318 XL (Figure 5-1).

An Aqueous Cleaner involves several cleaning processes and considerations:
1) First, determine if a cleaning agent is necessary for your
process. The mixture of cleaning agent to water is normally in the range of 3 to 30 percent. One way to determine if this percentage is correct is to use a refractometer.

2) The PCB is then pre-washed to help remove a majority of the contamination prior to entering into the wash chamber. An increased amount of physical space between the pre-wash and the wash will facilitate the removal of contamination in the wash chamber. The pre-wash is critical in a water-soluble application. OA fluxes contain organic elements (primarily sodium), which have a tendency to foam if not handled properly. The pre-wash runs at low pressure (30-40 PSI) and flows approximately 3-6 GPM of either tap water or water fed from a wash pump (cascading system). In an OA process, the majority of the flux is removed in this stage. After the water is applied to the PCB it should be directed to a waste water system or to a drain. It is extremely important to avoid recirculating this water in order to prevent excess foaming. Additionally, there must be enough physical space between the pre-wash and wash to prevent the organically filled water from “dragging” into the wash chamber. The high pressure recirculation of the wash pump will create a tremendous amount of foam, reducing the wash chamber’s ability to properly remove the contamination, and could possibly damage the pump itself. A pre-wash in an RMA/chemical application is used to apply the “cleaning agent” to help loosen the soils, allow extra time to expose the contamination, and aid in the wash chamber’s removal of the contamination. The pre-wash and wash tanks can and should be shared. Foaming is generally not an issue with these types of fluxes.

3) After the pre-wash, there is typically an isolation chamber (known as a dead zone). Some cleaning manufacturers consider this a part of the pre-wash chamber.

4) Next, there is the wash spray, which is traditionally a high pressure, high flow and heated chamber where all of the flux contamination should be removed. It generally contains 3-7 spray wands on the top and bottom, depending on the supplier. The longer the chamber and the more spray wands, the more exposure the PCB will have to the water. Most cleaning manufacturers recommend pressures and flows between 90-115 PSI at 50-80 GPM to effectively clean underneath fine pitch, BGA and micro BGAs. The correct combination of pressure, flow and length allows for faster throughput, and conveyor speed can be increased with an increase in flow. Also, increased pressure allows the spray to penetrate underneath tight clearance areas.

5) The isolation spray chamber’s purpose is to “isolate” the wash water and the rinse water. Isolation spray is not necessary in a straight water system. If the wash water is carried over into the rinse water, recontamination may occur, thus lowering the cleanliness level. If the wash water contains a cleaning agent, it can redeposit the cleaning agent onto the PCB, causing other contamination. If the rinse and final rinse water are closely looped, they will almost instantaneously exhaust the DI and carbon resins. It should also be noted that air knives should be placed before and after the isolation spray.

6) The rinse spray is a process used to completely remove contamination. This step uses a cleaner level of water, as this water cascades into the wash chamber. In an RMA process, this step is used to remove the cleaning agent from the PCB. It generally applies nearly the same pressure and flow as in the wash chamber but may have less spray wands. The water from this chamber cascades into the wash chamber.

7) Final sprays, depending on belt size, flow DI water at 3-5 GPM and 30-60 PSI to remove any possible remaining contamination. DI water acts as a cleaning agent by attaching to both positively and negatively charged ions. If the ions are still present, the DI water absorbs them as it passes over the product. The DI final rinse also lowers the surface tension, helping the water to move more freely and aid in the drying process.

8) The next zone is the drying chamber. The complete removal of the water is critical, as even good quality DI water can leave water spots and create contamination. A completely dry board is important when going directly into testing or conformal coating. Most cleaners use turbine style blowers with air knife technology that effectively removes all water. While some do offer infrared (IR) panels, it is not recommended in most applications. Using IR panels, the water is evaporated and often baked on, leaving water spots and possible contamination. Turbine blowers with air knife technology act as a squeegee to remove the water, leaving no water spots. The air is generally applied to the PCB at an elevated temperature (120-150oF) to help increase the “flashing” off of water.

The EMPF is currently using the Technical Devices Company’s Model 318 XL for numerous pilot PCB production runs. This cleaner recently assisted the EMPF in resolving a customer issue concerning the process used to clean PCB assemblies with a water soluble flux. The customer was interested in determining the levels of contamination and the effect of cleaning the assembly on those levels. Six identical, populated PCB assemblies were provided for testing. Three PCB’s were assigned to group one, and three were assigned to group two. Group one was only tested with an ion chromatograph. Group two was first cleaned in the 318 XL, then tested with the ion chromatograph. Fluoride and Chloride contamination levels decreased significantly after washing, and both levels were well below recommended guidelines for surface mount assemblies. After reviewing the test results, the customer initiated a process requirement for aqueous cleaning of their PCB assemblies.
A second batch of more than 500 PCB assemblies were successfully cleaned and immediately placed into service.

If you would like to see a demonstration of the Technical Devices Aqueous Cleaner, please call Jeff Stong at 610-362- 1200, extension 224.