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Printed circuit boards (PCBs) are critical components in many medical devices. Prior to shipment to the OEM, PCBs must undergo a thorough cleaning process to remove excess solder, rosins, and other contaminants at the end of the manufacturing cycle. If not cleaned properly, PCBs — especially those with complex configurations — may ship with unwanted contaminants.

Control panel of an Elmasonic P series ultrasonic cleaner. Controlling the cleaning time and temperature are essential to maintaining preferred PCB procedures.

A guidance document advises medical device OEMs that PCBs should meet a certain standard, noting “...a potential supplier of electronic circuit boards is required to provide circuit boards at a certain cleanliness level (minimizing reactive residues) to avoid reliability and performance issues associated with residue remaining from the soldering process.”1 In the last several years, FDA has become even more concerned about supplier management as critical components such as PCBs have continued to fail, leading to recalls and other issues. Risk management, including understanding how the supplier is cleaning the PCB, can help device manufacturers protect themselves against failures due to inadequately cleaned PCBs.

Ultrasonic cleaners have proved highly effective in cleaning PCBs, and the process is far superior to manual processes that involve soaking and scrubbing using high purity alcohol, flux removal sprays, and biodegradable solvents. By contrast, these time-consuming manual processes risk damaging delicate components and frequently fail to completely remove contaminants. This article explains why requiring ultrasonic cleaning can be an important factor when evaluating PCB suppliers. It describes elements that contribute to properly employed procedures and presents questions to ask regarding equipment, cleaning solution chemistries, and the care and maintenance of equipment.

How It Works

Ultrasonic sound is generally defined as sound wave frequencies above the range of human hearing, generally 20,000 cycles per second (20 kHz) and higher. Ultrasonic cleaning is performed when the implosion of billions of minute vacuum bubbles against surfaces of objects immersed in an ultrasonic cleaning solution quickly and safely strips away contaminants on those surfaces. Cavitation, the formation and collapse of these bubbles, is produced by the ultrasonic sound waves passing through the liquid. The sound waves are in turn produced by the high-frequency vibration of generator-powered transducers bonded to the ultrasonic cleaning tank.

Ultrasonic cleaner transducers determine the ultrasonic frequency generated for the cleaning process. Ultrasonic cleaners can be specified to operate at frequencies such as 25, 37, 45, and 80 kHz or higher. Some models operate at dual frequencies. The point to keep in mind is the higher the frequency, the (relative) smallness of the cavitation bubbles. Lower frequencies produce more vigorous cleaning and are usually used for heavily soiled parts. As the frequency increases, bubbles get (relatively) smaller for gentler cleaning and with better ability to penetrate tiny cracks, crevices, and surfaces typical of delicate PCBs used in manufacturing medical devices.

Bugaboos Dispelled

Removing cleaned PCBs. At IDC, a 5.5 percent dilution is prepared by combining 13 quarts of water and 0.75 quart of Elma Tec Clean A1 in the cleaning tank.

Early concerns about using ultrasonic cleaners for PCBs related to water damage and to unmodulated (fixed frequency) operation creating harmonic vibrations that could shatter PCB components. These concerns no longer apply when the proper cleaning solution is employed in cleaners equipped with what is called a sweep mode, which provides a slight continuous variation in the unit’s frequency. Another benefit of the sweep mode is a more uniform distribution of cleaning action throughout the bath.

Several water-based biodegradable formulations are available for use in cleaning PCBs. However, boards that have electromechanical properties like relays, as well as solid-state relays and optocouplers, may not be suitable for these solutions. It is best to look at the manufacturer’s data sheets to verify that the process is acceptable. The case study below indicates that trial and error exercises may prove useful in selecting a correct formulation. Experimenting on obsolete or broken boards can help develop and refine the procedures. A discussion with cleaning solution suppliers is also helpful.

The Nuts and Bolts

It is important to request an ultrasonic cleaner that offers the sweep mode to provide safe, uniform distribution of cleaning energy throughout the bath. Ultrasonic frequencies of 37 kHz and above should be considered. Dualfrequency cleaners operating at frequencies such as 37 and 80 kHz allow manufacturers options in developing the most effective processes for their PCB cleaning procedures.

Another specification point to consider is ultrasonic power. Without getting into the nitty-gritty, more power usually indicates faster and more effective cleaning, but more power is not always better. For example, too much power can damage electronic parts and other delicate items. For cleaning extremely sensitive items, equipment with adjustable power allows the PCB manufacturer to experiment to select the best power for a given PCB, or to accommodate a variety of PCB configurations.

It is critical that the supplier has a cleaner that accommodates the OEM’s PCBs. While this seems to be a “no brainer,” it is essential when it comes to specifying cleaning tank dimensions and, equally important, cleaning basket dimensions (which are slightly less than tank dimensions). The baskets hold the PCBs in a vertical position without crowding or board-to-board contact and must be of sufficient size to allow total immersion in the cleaning solution. This spec is called working depth and is the distance between the bottom of the basket and the surface of the cleaning solution. If this information is not available on the spec sheets, ask the manufacturer to supply it.

Support Racks

PCB support racks are a good way to aid positioning boards in the bath. Cleaning time and temperature controls are essential for establishing and maintaining preferred PCB cleaning procedures. Both of these are influenced by the condition of the PCBs and the recommendations provided by cleaning solution manufacturers. Cleaners equipped with heaters will cut the time needed to reach the recommended cleaning temperature, but it is important to note that the process of ultrasonic cavitation creates heat and warms the solution. Excessive heat can damage PCBs or subject them to thermal shock during rinsing. A useful accessory is a cooling coil. Check to see whether the supplier has a unit with a timer and automatic cutoff at the end of the timed cleaning cycle.

PCB Cleaning Cycle

Cleaning tanks have a fill line to indicate the maximum level of cleaning solution. The supplier should fill the tank half way with water, add the correct amount of cleaning solution concentrate for a full tank, and then continue adding water to the fill line. The unit is turned on to mix and degas the solution. Degassing fresh cleaning solutions removes trapped air that interferes with cavitation. Degassing time depends on solution volumes but generally takes 10–15 minutes. Some units are equipped with a degas mode that can speed the operation.

It is important to note that PCBs will cause solution displacement. An experienced PCB supplier will know how to accommodate displacement. The point to keep in mind is that an overfilled or underfilled tank should be avoided. Once the solution is prepared, it is ready to clean the PCBs. The operating parameters are set and the unit is turned on. The PCBs are placed in the basket or rack and lowered into the solution. The timer is set. At the end of the cycle, the PCB is removed and inspected. If it is a go, the boards are rinsed with deionized water to wash away cleaning solution residues and allowed to dry.

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