|A publication of the National Electronics Manufacturing Center of Excellence||June 2003|
Michael D. Frederickson
Harmful residues and contaminants are separated into two main categories: ionic and nonionic. Ionic residue can be described as residue that contains molecules or atoms which are conductive when in solution. With the addition of moisture, ionic residues can disassociate into either negatively or positively charged species and increase the overall conductivity of the solution. Some of the most common sources of ionic residue include:
Non ionic residues are not conductive and are usually organic species that can remain on the PWB after board fabrication or assembly. These include:
While both ionic and non ionic contamination can impact the operation and reliability of the device on which they are present, the effects of ionic contamination is of greater interest to most PWB manufacturers. A higher figure of failures is associated with ionic contamination than its nonionic counterpart. Corrosion and dendrite growth (Figure 1) are the two most common failure modes resulting from ionic contamination. Either can lead to shorts and opens in an electronic circuit, particularly in fine pitch applications or assemblies with high component packing density.
Nonionic (nonpolar) residue will lead to unwanted impedance because of its insulative properties. This is an issue particularly where plug-in contacts or connectors are utilized. In addition to acting as an unwanted insulator, non ion contamination on an assembly can cause poor adhesion of solder mask and conformal coating, physical interference with moving parts, encapsulation of ionic contaminants, and retention of foreign debris.
Due to dissimilarity in the fundamental characteristics between these two forms of contamination, different test methods are required to test them. The most prevalent method of measuring the degree of ionic contamination is to measure Resistivity of Solvent Extract (ROSE). ROSE testing is also known as Solvent Extract Conductivity (SEC). The concept of the ROSE methods is quite simple; as the level of ionic contamination increases, the resistivity of the solution decreases.
ROSE tests can be either static or dynamic. In static testing a solvent is deionized to a stable resistivity. The PWB is added to the test cell and soluble ionic residues drive the resistance down. The change in resistivity is then used to calculate the cleanliness of the PWB. Dynamic testing is performed similarly to the static test with the exception that the solution is deionized during testing and change in resistivity is plotted against the time it takes to deionize the solution back to the original starting resistance. IPC-TM-650, method 2.3.26 and 18.104.22.168 outlines in detail the procedure for performing ROSE testing.
Simple automated versions of ROSE testing are used by PWB manufacturers. The Omega Meter, Ionograph, and ZeroIon are used by a number of assembly houses for quality assurance purposes. They compare the conductivity of the extract solution before and after testing. The response is reported as a sodium chloride (NaCl) equivalent per unit area. These devices were designed to be used with RMA fluxes and may act erratically with PWBs fabricated with low residue (no-clean) fluxes.
|These tests are also restrictive in the following ways:
1. they measure only ionic contaminants
2. they measure the contamination washed off the assembly, not left residing on it
3. they measure an average contamination for the assembly and do not define the exact source of the contamination
Results from one piece of equipment, the OmegaMeter, Ionograph, or ZeroIon, are not equivalent to another. For example, an Ionograph result of 10µg of NaCl eq. /in2 is not the same as 10µg of NaCl eq. /in2 on a ZeroIon. The MILP-551100 document can be used to equilibrate the results obtained from these test methods.
Ion Chromatography (IC) is the most common tool for precision testing and process base lining. This system can quantify and identify specific ionic species that are present on an electronic device. The most common test method is the IPC TM-650 2.3.28. The device is placed into an ionically-clean bag and is immersed in an extract solution of 75% alcohol and 25% deionized water at 80°C for one hour. This is a much more rigorous extraction method than the methods used by automated ROSE equipment. The IC then separates and detects each individual ion for which it was calibrated. The EMPF uses a Dionex DX500 system to detect and measure fluoride (F-), chloride (Cl-), bromide (Br-), nitrate (NO3-) nitrite (NO2-), phosphate (PO4-), sulfate (SO42-), and weak organic acids (WOAs). Results are reported in µg/in2. IC has a major advantage over the quality control automated ROSE techniques because it allows a trained scientist or engineer to determine the source of the ionic contamination. For example, a customer using a low-residue flux observed intermittent failures. IC testing detected elevated levels of chloride (22µg/in2). Testing of the bare boards used for these assemblies also showed high levels of chloride (27µg/in2). From this, it was determined that the fusing fluids used during the HASL process remained on the boards, carrying through to the final assemblies.
It is important to mention that there are no published acceptability standards for the degree of ionic cleanliness. While the EMPF can recommend upper control limits for ionic cleanliness, the board layout, operating environment, reliability requirement, and manufacturing process must be taken into account when considering ionic cleanliness. Factors such as conductor spacing, cleaning quality, and leakage current tolerances greatly influence ionic cleanliness acceptability levels.
Surface Insulation Resistance (SIR) testing is a test method that measures the effect of contamination remaining on the board. It consists of an electrical test that measures a change in electrical current over time and is typically performed at elevated temperatures and humidity levels. One of the most common tests is conducted using IPC TM-650-22.214.171.124. This test is intended to characterize fluxes using a standard comb pattern at 85% relative humidity and 85ºC. The presence of contamination lowers the insulation resistance of the material between the conductors. (Figure 2)
Other than simple optical inspection, there are no simple automated methods of measuring non-ionic residue. The most common analytical method is Fourier Transform Infrared (FTIR) spectroscopy. Infrared light is applied to the sample and the resulting spectra are compared to a commercial database. IPC-TM 650 2.3.39 describes the FTIR method for identifying organic contamination on the surface. The EMPF compliments its own database with a nationwide internet database. UV-Vis spectroscopy (IPC-TM-650 2.3.27) is frequently used to identify residual rosin. Scanning Electron Microscopy (SEM), with energy dispersive x-ray (EDX) analysis, is employed to identify heavier (higher atomic mass) elements present while Auger analysis identifies lighter (low atomic mass) elements.
Cleanliness of a PWB should be addressed periodically as a quality control tool whenever problems arise or whenever changes in process or materials are implemented. Alterations in cleaning methods, saponifier chemistries, and paste chemistry are the process changes that warrant the most attention to cleanliness testing. However, even the smallest modification to the board design could necessitate a change in the cleaning process and require cleanliness testing. The EMPF laboratories are well equipped to help with cleanliness testing and possess a unique combination of R&D tools, analytical testing facilities, reliability assessment capabilities and production/manufacturing equipment.
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