Materials Characterization

Why HPLC Is Important

IPC requires the cleanliness test for bare and assembled PWBs for the determination of the residues of chloride, bromide, and sulfate to avoid corrosion and dendrite growth. The related test method is IPC-TM-650, Method 2.3.28 “Ionic Analysis of Circuit Boards, Ion Chromatography Method” [9] and, it requires the column AS4A-SC, which is used by CALCE.

Sulfate:

In PWB manufacturing, sulfate combined with water from an aqueous cleaning process and subsequent reflow preheats etches open the barrels of holes in a circumferential manner. The sulfate remains in the holes as a residue from an organic solder preservative (OSP) coating process, which contains sulfuric acid as a microetch agent. The problem does not surface until the hole diameters become very small relative to the depth (large aspect ratio). CALCE can detect sulfate*.

Bromine:

Gold on aluminum produces a galvanic couple that can accelerate the corrosion process by providing the driving force for the aluminum oxidation reaction. Bond corrosion in gold-aluminum bonds can also result from the liberation of bromine from brominated flame retardants at high temperatures. The mechanism proposed by Ritz [13] included the liberation of bromine from methyl bromide (CH₃Br) or of hydrogen bromide (HBr) from the high-temperature breakdown products of the resin. The bromide ion (Br-) reacts with the aluminum in the gold-aluminum (Au₄Al) intermetallic phase, forming aluminum bromide (AlBr₃) and gold. The aluminum bromide oxidizes, and this oxidation reaction provides the driving force until the Au₄Al intermetallic is consumed. The liberated bromide ions sustain the reaction and can completely corrode the bond pad [13]. Bond corrosion may not directly result in a failure, but it can increase the electrical bond resistance to a level that renders the device non-functional [14, 15]. CALCE also has the capability for bromide anion determination.

Chlorine:

The ions most commonly found on the die surface and most likely to form electrolytic solutions, triggering the corrosion process, include phosphorus; halides (especially chloride and bromide); alkali (sodium) emitted from diffusion ovens, glass containers, and the hands of humans; and sulfur emitted from rubber bands and cardboard separators used to bundle components in storage, can cause sulfide corrosion of exposed copper-silver eutectic braze alloys [13]. Of all contaminants, chloride ions are the most significant source of corrosion, readily combining with the most common metals (copper, lead, tin) used in electronics. Through an autocatalytic corrosion process, metal chlorides continuously regenerate free chloride ions to sustain the corrosion process [8]. CALCE has the capability of chloride determination.

Copper, which is used in most PWB designs, is extremely susceptible to corrosion from chlorides and sulfides. Surface corrosion can increase surface film resistance and produce by-products that accelerate wear. Another corrosion failure risk in PWBs is the reduction of insulation resistance between adjacent conductors. Insulation resistance is lost by the formation of conductive bridges, and corrosion plays a significant role in this area. Failure under this condition may be attributed to dendrite growth or conductive filament formation [1].

The presence of halides, such as chloride and bromide, at high levels in electronic packaging materials has been shown to have an accelerating effect on the occurrence of electrochemical migration (ECM). This effect has been seen at various packaging levels. The time-to-failure in plastic encapsulated materials can be directly related to the chloride content in the epoxy-molding compound (EMC) [2]. Very little chloride is required to cause considerable damage to the chip. In addition, copper oxidizes relatively quickly. In anodic corrosion, the metal acting as the anode undergoes oxidation and produces metal ions. Anodic corrosion of aluminum metallization is frequently observed when chloride contamination is present. Unfortunately, chloride ions are common among the chemicals used in processing electronic components. Chloride ions are a by-product of the epoxy resin chemistry of encapsulation compounds, and can be carried as a trace impurity.

The amount of ionic impurities is influenced by the cleanliness of the resin used, the substance chosen as curing agent, and other additives. The mix ratio between resin and curing agent may also be of concern. Lambert states the content of chloride ions increases dramatically if the weight percent of resin is below 40%. Below this mix ratio the excess of curing agent is easily hydrolysable which explains the high ionic impurity. [5]

Fluorine:

For the evaluation and acceptance for polymeric adhesives according to MIL-STD-883, Method 5011 [10], it is necessary to report extractable chloride, sodium, fluoride and potassium ion levels. CALCE has the capability for chloride and fluoride ion determination.

On pre-wired connectors in factory sealed bags, corrosion was noted on gold crimped pins which had not yet been installed into a connector. The corrosion appeared much more frequently on the white Teflon wires than for colored Teflon wires. Using ion chromatography analysis on many different samples, the corrosion was shown to correlate to levels of fluoride that often exceeded 50–75 micrograms per square inch.  In areas where no corrosion occurred, the fluoride levels were much lower than this. Fluoride is one of the most electro active residues known, and is much more corrosive than chloride. The ‘threshold’ level of fluorine required to initiate the corrosion process is unknown [6]. CALCE has the ability to detect fluoride ion.

Corrosion of aluminum within an integrated circuit (IC) can proceed at greatly accelerated rates in the presence of water by anodic corrosion when certain ions (such as chloride) are present at positively biased conductors [3] (hydrolysable ions such as chlorine, fluorine, sodium and ammonia in the presence of moisture can lead to corrosion of aluminum metal traces on an IC.

References:

1.  Osterman, M. “Reliability and Performance of Advanced PWB Assemblies,” High Performance Printed Circuit Boards, Harper, Charles A., editor, McGraw Hill, New York, NY, 1999.

2. Pecht, J. and Pecht Michael, Long-Term Non-Operating Reliability of Electronic Products, Boca Raton, FL, CRC Press LLC, 1995.

3. Hagge, John K. “ROBOCOTS: A Program to Assure Robust Packaging of Commercial-Off-The-Shelf (COTS) Integrated Circuits”, International Journal of Microcircuits and Electronic Packaging, IPMAPS 2000, Boston, September 2000 vol. 23, no. 4 pp. 424-434.

4. R. Hunadi, et al, “A New Ultra-High Purity, Electrically Conductive Epoxy Die Attach Adhesive for Advanced Microelectronic Applications”, Proc. ISHM, November 1985, pp. 402-407.

5. Lambert, Francois "H20 E Mixing ratio influence on over-all performances" Epotency France 1987.

6. Cabourne, June, “Case 33,” http://www.residues.com/pdf/Cases33.PDF. Accessed on 31 January 2002.

7.  “Designing for Reliability,” http://www.semicon.toshiba.co.jp/eng/prd/comminf/pdf/chap02.pdf. Accessed on 31 January 2002.

8. Maria L. Peterson, Robert J. Small, Gordon A. Shaw III, Zhefei J. Chen, and Tuan Truong, “Investigating CMP and post-CMP cleaning issues for dual-damascene copper technology”, EKC Technology

9. IPC Publication IPC-TM-650 2.3.28, “Ionic Analysis of Circuit Boards, Ion Chromatography Method,” Northbrook, IL, January, 1995.

10. Military Standard MIL-STD-883 Method 5011, “Evaluation and Acceptance Procedures for Polymeric Materials,” Columbus, OH, 31 October 1995.

11. Cieslak, W.R. Failure Analysis of 24-Pin Leaded Chip Carriers. Electronic Packaging and Corrosion in Microelectronics, (1987), 217-220.

12. Chen, F. and Osteraas, A.J.  Electrochemical Dendrite Formation During Corrosion of Connector Leads.  Electronic Packaging and Corrosion in Microelectronics, (1987), 175-178.

13. Ritz, K. N., Stacy, W. T., and Broadbent, E. K. The Microstructure of Ball Bond Corrosion Failures. 25 th Annual Proceedings of the IEEE Reliability Physics Symposium (April 1987) 28-33.

14. Paulson, W. M. and Lorigan, R. P. The Effect of Impurities on the Corrosion of Aluminum Metallization. 14th Annual Proceedings of the Reliability Physics Symposium, Las Vegas, NV (1976) 42-47.

15. Iannuzi, M. Bias Humidity Performance and Failure Mechanisms of Non-Hermetic Aluminum SIC's in an Environment Contaminated with Cl2. 20th Annual Proceedings Reliability Physics Symposium, San Diego, CA (1982) 16-26.