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.
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.
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.
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.
Cleaning Solution Maintenance and Disposal
The large majority of today’s cleaning solution formulations are biodegradable and can be disposed of in sanitary drains, depending on local regulations. Cleaning effectiveness should be maintained by skimming off contaminants, including solder flux rosins that float to the surface, and putting them aside for later disposal.
Larger ultrasonic cleaners may be equipped with skimmers and weirs to direct floating contaminants to collection containers. They may also be equipped with filtering systems that trap suspended and settled contaminants, returning treated solution to the tank.
Eventually the solution must be replaced. The tank is drained and should be thoroughly cleaned according to the manufacturer’s recommendations. The PCB supplier should be especially aware of solid contaminants that have fallen to the bottom of the tank. If allowed to remain, these will, over time, serve as drills that may cause holes to develop, necessitating a tank or complete unit replacement. The PCB manufacturer should never use an abrasive cleanser on an ultrasonic cleaner tank.
Case Study: IDC
Ohio-based Independent Digital Consulting (IDC) builds, tests, and delivers circuit board assemblies. This case study relates how the company developed its optimum PCB cleaning process through experimentation. Owner Tom Schurr notes that the importance of clean circuit boards in medical applications cannot be stressed enough and explains why.
“In many medical designs, there are high impedance circuits that are very sensitive to stray current being introduced by other nearby circuitry,” he says. “Even if creepage and clearance distances are observed, any residue left on the circuit board after cleaning will provide a path for leakage currents to cause intermittent or faulty operation. Residue will provide conduction paths for high-frequency digital signals to leak into sensitive analog circuitry, which can cause faulty readings or worse.
“No residue can be left on the assembly, either on the board surface or on the leads of the components,” he explains. “The most difficult place to clean is between the board surface and the portion of the components that rests on the board. When following proper procedures, ultrasonic cleaning reaches those locations when other techniques will not. When the board is not properly cleaned, the soldering process can leave behind a surface film that may be initially benign to conduction and leakage, but over time will absorb moisture and provide those undesirable leakage paths. In addition, an improperly cleaned board may not allow environmental coatings to adhere to the board.”
For IDC, Schurr uses the heater-equipped dual-frequency Elmasonic P180H model with internal tank dimensions of 12.9 in. long, 11.8 in. wide, and 7.9 in. deep, with a liquid capacity to 5 gallons. He chose this unit because the three lines in the P series offer adjustable ultrasonic power that can be set from 30 to 100 percent of the P180H unit’s 330-W effective power. He also wanted the option of using a less-aggressive ultrasonic frequency of 80 kHz in addition to 37 kHz. “The P line also provides the critical sweep function to prevent harmonic vibrations that could damage delicate PCB electronics, and a degas mode to drive off cavitation-inhibiting entrapped air,” he says.
Specifying ultrasonic cleaning solution chemistry is as important as the equipment. Today’s biodegradable cleaning solution concentrates are formulated for a variety of cleaning challenges. In this case, the mildly alkaline concentrate Elma Tec Clean A1 proved the best choice for cleaning electronics such as PCBs. This concentrate should be diluted to 3–10 percent with water.
“For our operations, a 5.5 percent dilution was prepared by combining 13 quarts of water and 0.75 quart of A1 in the cleaning tank,” Schurr says. The unit is started, and the degas mode is activated to thoroughly mix and degas the solution. This step applies to each time fresh cleaning solution is prepared.
IDC’s PCB Cleaning Steps
Boards are carefully positioned in the cleaning basket so that they do not contact each other. The ultrasonic cleaner is readied by turning on the power, setting the frequency to 80 kHz, activating the sweep mode, setting the thermostat to 40 °C (104 °F), and setting the power to 30 percent. Set and actual values are displayed on the control panel. When all is ready, the basket is immersed in the solution and the timer set for 7 minutes.
“Selecting the correct power level was achieved by experimentation during operational tests,” Schurr explains. “We wanted gentle but thorough cleaning, and this is optimized by keeping the level below 10 W per quart. Since ultrasonic cavitation produces heat, 40 °C was specified so the boards do not get too warm and are not subject to thermal shock during the rinsing operation.”
Schurr adds that the 7-minute cleaning cycle may vary depending on the number of boards cleaned and their condition. “At the end of the cycle, the basket is removed and the boards are rinsed using deionized water and then air dried under a fan,” he says, cautioning that while the process in general is excellent for PCB cleaning, there are exceptions as mentioned earlier.
A Caution on Solvents
This discussion centers on water-based biodegradable cleaning solution formulations. Solvent-based cleaning solutions for water insoluble fluxes may require special precautions. For example, nonflammable solvent-based solutions can be used very effectively on water-insoluble fluxes. However, if these are used in filter-equipped tanks, it is essential that the filtering system is compatible with the formulation. If flammable solvents are used, an explosion-proof ultrasonic cleaner must be used and must comply in what is termed a hazardous area. This is a subject unto itself and calls for consultations with suppliers of explosion-proof ultrasonic cleaners.
This article was written by Dr. Rachel Kohn, Director and Cofounder of Tovatech, Maplewood, NJ. For more information, Click Here .
- GHTF/SG3/N17:2008, “Quality Manage ment System – Medical Devices – Guidance on the Control of Products and Services Obtained from Suppliers,” Global Harmonization Task Force, 2008.