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Ion chromatography (IC) is an analytical laboratory technique that uses the principle of ion exchange to separate and quantify organic and inorganic ions. Historically, it has remained in the biological arena where this analysis method was used to separate amino acids. 1 However, in the electronics manufacturing industry; it is an important tool for determining specific levels of common ionic contaminants from flux residues.
Prior to the use of ion chromatography, the technique known as Resistivity of Solvent Extract (ROSE) was used to quantify the ionic cleanliness of PWBs and assemblies. ROSE measures the total conductivity of the extract solution and relates the measured conductivity of the solution to a known level of ionic contaminants in units of micrograms Sodium Chloride per liter of extract. Ion chromatography has the advantage over ROSE because it quantifies the breakdown of the sources of ionic contamination, in this case: fluoride (F-), chloride (Cl-), nitrite (NO 2 -), bromide (Br-), nitrate (NO 3 -), phosphate (PO 4 - 3 ), and sulfate (SO 4 - 2 ).
Theory
Ion chromatography incorporates a mobile phase and stationary phase. The mobile phase in this case is usually water and some buffer mixture. Buffers are mixtures of acids and their salt or bases and their salt, and maintain the pH of solutions. In ion chromatography, pH is a critical parameter and must be kept constant. The stationary phase is the column which contains an active resin. The unknown is usually in the form of a liquid sample that is injected onto the column. The sample is pushed through the column by the force of the constant flow of the mobile phase. As the sample contacts the column, the dissolved ions in the sample will have an affinity for the column and replace less retained ions like those which make up the buffer. This exchange process is continuous, however, and the length of time that the various ions retain themselves on the column is what delays their travel through the column. Since each ion has a different affinity for the column, some will spend less time while others will spend more time in the mobile phase. The fact that each ion has a different residence time in the mobile phase allows for its separation. Eventually, each ion will come out of the column and be detected by the conductivity detector. The result is a peak and the area under each peak represents the relative amount of each ion. When compared against known standards, the amount of each ion can be determined.
Practical Information
The industry-standard procedure used in electronics manufacturing is IPC TM-610 2.3.28 "Ionic Analysis of Circuit Boards, Ion Chromatography Method". The analysis procedure is summarized below:
Bare PWBs, PWAs or components are extracted with a 75/25 Isopropyl alcohol/water solution using an 80°C water bath for 1 hour. The extracts are analyzed against known standards to confirm the presence of and to quantify each anion in units of µg/ml. The total surface area of the sample is determined and the final results reported in µg/inch 2 .
The procedure is applicable to bare PWBs, PWAs or individual board components (resistors, capacitors, transistors, etc.) It should be noted that the analysis is also applicable to board washings or cleaning residues. In that instance, the extraction step with 75/25 Isopropyl alcohol/water is not necessary; the samples can be analyzed directly if in a liquid form.
Some considerations to the individual ions when analyzing IC data:
- High levels of bromides observed are most likely from fire retarding chemicals in the solder mask, component packages, and board substrate. It is possible that the presence of bromide ions could be a product of depaneling the boards. FR-4 often contains bromide residues from the fire retardant added to the epoxy polymer.
- For high chloride levels on the PWB, the two most likely sources are poor handling (e.g. sweat or salt from someone's hands) and/or improper/incomplete cleaning after bare board fabrication. A chloride amount of greater than 6 mg/in 2 present on the cleaned board is considered moderate, and may cause a concern in a high reliability and/or a high humidity environment by causing dendrite growth, leakage current, or corrosion.
- Sulfate peaks are likely due to an organic acid present in flux residue.
- Surface insulation resistance (SIR) testing can also aid in determining the impact that the contamination levels observed may have on long-term reliability. ASIR test and/or an electrochemical migration (ECM) test can be performed at ACI to determine the impact of the ionic residues on the surface of the production board. If the board fails the SIR or ECM test, the production process of these boards needs to be examined in terms of cleaning issues.
ACI can perform both ion chromatography and surface insulation resistance testing to determine residual ion contamination on electronic circuit boards and components. Knowledge of this is important to ensure proper reliability and conformance to specifications. Call the EMPF Helpline (610-362-1320) for more information about your Ion Chromatography needs.
References
1Practical High Performance Liquid Chromatography, 3rd; Veronika R. Meyer p.172.
As an example, an electronics manufacturer wanted to determine the levels of ionic contaminants on populated printed wiring assemblies (PWAs). The customer supplied boards and sample of low solids VOC-free no-clean flux for wave-soldering were examined as per ACI lab procedure and IPCTM- 650 2.3.28. Briefly, the PWAs were extracted in a 75% isopropanol/ 25% water at 80oC for one hour. The extract was then examined using a Dionex DX-500 ion chromatograph (IC) for fluoride, chloride, nitrite, bromide, nitrate, phosphate, and sulfate anions. ACI's maximum recommended amounts of fluoride, chloride, bromide, nitrate, and sulfate are 5, 10, 15, 15 and 20 µg/in2 respectively. These limits were developed in conjunction with industry leaders and apply to typical component packages on FR-4 or a like substrate.
Results show that all the anions tested for were well below the recommended levels except for sulfate, which was two to five time the recommended value (<20g/in2). Testing of the flux by ion chromatographic analysis showed the sulfate peak in the chromatogram is likely due to an organic acid within the flux. An authority in the area of flux and flux residues stated that this should not be a reliability issue as sulfur is typically bound tightly in the flux residue chemistry. Knowledge of the tolerance of the materials used on the board should be kept in mind however.
One example that readily stands out are common resistors that have silver/ palladium terminations. There may be certain environments that allow sulfur bound within the flux residue to meet the affinity that silver has for sulfur.
This example shows a situation where IC analysis has limitations on only determining the amounts of ionic residues remaining on a PWA. To supplement the testing, surface insulation resistance (SIR) can aid in determining the impact that the observed contamination levels may have on long-term reliability. |
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