Is it clean? Why do you want to know? Part I: do you sometimes battle with your customers over cleanliness?

Q: Is there a standard my company can use to demonstrate that both our no-clean and cleaning processes deliver soldered assemblies with "good cleanliness" and put all this questioning to rest?

A: Unfortunately, the answer is probably closer to "no" than it is to "yes" but don't panic. The answer depends on when and why you are testing! A good explanation of this caveat will take two columns to provide a meaningful answer--I'll tackle the cleaning process this month. Next month's column will discuss process control testing.

Establishing a Robust Cleaning Process

The first, and perhaps the most important, task is to characterize a robust cleaning process capable of removing soldering process residues from soldered assemblies. To reliably access and remove residues left by the flux selected, the cleaning process must be defined and optimized to deal with the most complex component and assembly design packaging technology. Any other undesirable residual materials, which may be on the surfaces of the printed wiring board (PWB) and components, from supplier or in-house operations, are best controlled and dealt with before the soldering operation. If such contaminants are not dealt with early on, solder joint quality and reliable operation of the delivered device will be at risk. Following successful qualification, with the use of a cleaning process, a similar process characterization needs to be carried out with the no-clean process.

Incoming materials and components should be received clean and solderable. Although cleanliness is the responsibility of the supplier, establishing regular in-house auditing of incoming materials is wise and will establish the need for quality in the mind of the supplier, as well as your shop operation. After acceptance, materials must be maintained in a clean and solderable condition--while in the stockroom and on the manufacturing floor--until delivered to the customer.

Establishing and characterizing a robust soldering process, with or without a cleaning operation, requires going beyond basic process control testing referenced in specifications and literature. Initial qualification of the process and periodic audits require additional and varied testing to demonstrate that a controlled manufacturing process exists and is maintained. Initial qualification of a clean process includes a review of the total manufacturing process. To optimize a cleaning operation, the thermal, chemical and physical force parameters of the cleaning process should not damage the assembly or degrade the operational characteristics of the product.

Evaluating the Process

A manufacturing process' cleaning capabilities are best evaluated with the use of the ion chromatography technique. This test can identify the presence and amount of specific ionic species on the tested assembly. Also of value is the surface insulation test (SIR), which exposes an energized sample assembly to a temperature and humidity environment while measuring operating characteristics, Each test is often conducted during qualification of production facilities, materials and processes.

Ongoing monitoring and control of the total process is accomplished with various test techniques capable of measuring and indicating whether the cleaning process, equipment and chemistry are capable of removing unwanted materials and process residues from produced assemblies. The most common technique used for quick evaluation of ionic contamination remaining on soldered assemblies is known as the solvent extract ionic conductivity test, often referred to as resistivity. This test provides an indication of the level of ionics remaining on an assembly. The test values exhibited vary with the type of equipment and test conditions used. The purpose of this quick, shop-conducted test is process control with an established, effective and monitored process.

An in-process ionic conductivity test does not necessarily indicate whether the ionic contamination present will compromise operation in the field. The in-process test simply indicates the level of ionic contaminates remaining on the assembly at the conclusion of the soldering and cleaning operations.

Send your process, technology or training question to [e-mail]. Please type "ASK LES" in the subject line and indicate your name and company or institute affiliation. All questions may not be answered.

Les Hymes is the owner of Les Hymes Associates, Surprise, AZ; [phone]; e-mail: [e-mail].