Electronic Assemblies: Cleanliness is Next to Reliability

[paul-voelker]

Cleaning electronic components and assemblies is essential to many industrial processes as a precursor to surface finishing, protect sensitive finishes, and ensure long-term performance and reliability. Contamination on printed circuit boards can be the result of flux, solder, adhesives, and solvents resulting in current leakage between the circuitry, promote dendrite growth, and increase the risk of corrosion.

 

Challenges in Cleaning Electronic Components

While appearing straightforward, cleaning electronic components is challenging. Back in the day, any components of any design were cleaned using chlorofluorocarbons (CFCs) and  hydrochlorofluorocarbons (HCFCs). These cleaning agents offered effective solvency, low surface tension (facilitating contaminant dissolution), and quickly evaporated to dryness leaving negligible residue.

The ‘90s explosive growth in electronic devices, miniaturization of components, and the ban on CFCs due to the discovery that CFCs and HCFCs were depleting the ozone layer created a new set of cleaning challenges. Introduction of smaller components raised design issues in addition to handling and reliability problems. New solder fluxes and pastes were developed to match the capability of new cleaning agent formulations resulting in new test methods to quantify a different set of analytes.

 

Anion Contamination of Drive Components

Anions are one of the possible contaminants of drive components and can originate from a range of sources: packaging materials, human contact, assembly environment, aqueous rinse solutions, solvents, adhesives, and lubricants to name a few. A comprehensive anion analysis of drive components prior to manufacturing can significantly reduce the incidence of corrosion and failures as in the case of hard-to-clean head-to-disk interfaces which have tight clearance tolerances.

An ionic cleanliness test measures the conductivity (or resistivity) of a sample which can be correlated to the quantity of ionic material present. Ionic contaminants are generally water-soluble salts, organic and inorganic acids. The anions most routinely monitored are fluoride, chloride, bromide, nitrate, sulfate, and phosphate. Acetate, formate, acrylate, methacrylate, benzoate, and oxalate are also of interest.

 

Measuring Ionic Contamination on Printed Boards and Assemblies

Standards and test methods that measure the level of extractable ionic contamination on the surface of printed boards are governed by regulatory agencies such as the Association Connecting Electronics Industry (IPC). In particular, the IPC-TM-650 Test Methods Manual provides a test procedure for measuring the level of extractable ionic contamination on the surface of printed boards and assemblies by ion chromatography.

The method describes a sample extraction procedure where the electronic component is placed in a extraction bag (a heavy duty pouch that is solvent and temperature resistant) and heated in a bath of 75/25 v/v isopropanol/water solution at 80^oC for one hour and then analyzed by ion chromatography.

Ion chromatography is ideally suited for identifying and quantifying specific anionic species present in an electronic device with sensitivity in the parts per billion levels and lower as described in this application note, titled, Monitoring for Trace Anion Contamination in the Extracts of Electronic Components (downloadable PDF).

Figure 1 below shows results from the analysis of the water extract from a disk drive spacer using two injection methods: a 1 mL direct injection method and a 5 mL preconcentration method. Results show both methods to be effective and vary according to the sensitivity requirements. The preconcentration method had the most sensitivity, but requires an additional concentrator column, more sample, and more time to load the sample. The 1 mL direct injection method (A) had good sensitivity without the need for a preconcentration method (B).

Analysis was performed on one of our Thermo Scientific Dionex ICS ion chromatography systems using a KOH eluent gradient with suppressed conductivity detection) and one of our ion chromatography columns (Thermo Scientific Dionex IonPac AS17 anion exchange column).

 



 

Do tell us of your experiences in measuring contamination on printed circuit boards: I look forward to hearing from you.