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www.us-tech.com PCB Cleaning: It’s Not All Black and White By Mike Jones, MicroCare Corp.
have to contend with and how can they measure cleanliness to ensure the device’s reliability? PCBs today are multilayered and highly com-
G
plex assemblies. Technology is advancing and the demand is growing for smaller and more intelli- gent devices. The miniaturization of electronics is making the management of faults even more critical to longevity.
Causes of Contamination There are two types of contamination,
ionic and non-ionic. Ionic contamination is usu- ally flux residue left behind during the manu- facturing of a PCB, or after the soldering process, and is usually inorganic salts or acids. This ionic residue contains molecules that are conductive when in a solution. If it is exposed to levels of moisture, the
ionic residue can split into negatively or posi- tively charged ions, increasing the overall con- ductivity of the solution. If left untreated, it will degrade and may lead to corrosion and dendrite growth causing leakage and short circuits. This will ultimately result in the failure of a device. Non-ionic contamination is a non-conductive
molecule that can remain on the surface of the PCB after assembly. The main culprits include rosin, oil, inert flux residue, and grease — all of which can cause circuitry problems. Ionic contamination causes the highest per-
centage of electronic device failures and can be the trickiest to eliminate. One of the common forms of ionic contamination is called “white residue.” These residues are usually metal salts, which are
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are stored, and even the weather. It is very hard to determine precisely what might be causing the problem. The most likely causes are the contamina- tion, the cleaning fluid or the process. To identify the source, it is important to first
look for any change that has recently occurred in what what is being cleaned. Was a process just changed to lead-free? Or, did a spool of RMA solder get slipped into a no-clean process? These materi- al changes can cause unexpected residues and cleaning problems. Next, look for any disparity between the
You Can Trust
uaranteeing the cleanliness of a PCB is key to the reliability of its circuitry. But, what kinds of contamination do manufacturers
the activators in fluxes or pastes, resulting from insufficient cleaning methods. White residue is one of the biggest annoyances
in the electronics manufacturing industry. There are dozens of different possible sources, including the boards, the cleaning fluid and fluxes, the people, the processes, the way the fluxes and solder paste
cleaning fluid and the contamination. If the residues are an even, smooth layer of white film across a large area, this usually indicates that the wrong solvent is being used for that flux. Or per- haps the fluid is reacting with no-clean fluxes and producing white residues. Finally, assess the cleaning procedure being
used. Very often, white residue problems are due to a process problem. Look for streaks or spots of white residues. These are the indicators of improper cleaning techniques.
Validate Cleaning through Inspection There are many ways to validate a clean-
ing process and measure the level of contami- nants. Some use visual evaluations as a bench- mark, while others use analytical testing, such as ion chromatography. The validation process is unique to the cleaning requirements of the type of contamination being removed and the final product. Visual inspection is the easiest method. If
there are no visible flux materials and no white residues, most people just call it “clean enough.” Although there are no quantitative results, it is
an important method to use alongside other processes. A cautionary note: some ionic contami- nant is not visible in optical inspection and can still be lurking around the components of the PCB. For this reason, it is important not to rely solely on this method. At the other end of the scale is surface insula-
tion resistance (SIR) testing, which concentrates more on the actual outcome of the cleaning being performed and less on the actual cleaning process itself. SIR testing specifically focuses on determin-
Continued on next page
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