White Residue: Good or Bad? -- Discover which cleaning protocol really works-and why.

A study was recently conducted to examine process residues remaining under micro ball grid arrays (BGAs), with ball counts above 1,000 I/O and water-soluble flux, after water and saponified cleaning is performed on boards. White residue was found on the surface of the board in the area where the BGA was attached. IPC 610 C inspection criteria states that such residues are a reject criteria unless proven benign.

Process History

A water soluble, run in nitrogen (full tunnel) solder paste was used in the study. Saponifier #1, a 5 percent from Vendor A, and saponifier #2, a 5 percent from Vendor B, were used in a standard, in-line aqueous cleaner at a 57.2 degrees C wash temperature. Boards were fabricated using an LPI solder mask and Entek 106A organic solderability preservative (OSP) board finish. The BGA board area and the components were tested in the same extraction bag. All integrated circuit (IC) testing was performed per IPC TM-650 2.3.28 ion chromatography test method.

Data Review

The assemblies were extracted in the area of the BGA only. The boards were cleaned using three different cleaning protocols-water only, saponifier #1 and saponifier #2. The bare board established the foundational cleanliness of the assemblies. Prior to cleaning, the assembly showed typical water-soluble flux residues. The white residue, observed below the BGA packages, is the remaining flux residue after cleaning. Ionic residues found are directly related to the remaining flux residues.

The level of chloride, bromide and weak organic acid (WOA), for the water-cleaned only solder paste, was high and posed great risk for electrical leakage and electromigration problems. Results showed that saponifier #1 and the water-cleaned only paste left high levels of flux residue on the surface of the board-below the BGA component. Saponifier #2 removed more flux residues ionically and visually and reduced ionic levels to well below recommended levels.

Answers to Your Questions

Q: What is the nature of the residue remaining under the microBGA?

A: Flux residues, with water-soluble flux activators, are conductive and corrosive if left in large enough quantities on the board.

Q: What amount of residue is acceptable?

A: If you can see residue, be concerned.

Q: Is the residue good or bad?

A: In the water-soluble flux case, the residue is bad and must be removed. If residue levels are above recommended limits, ambient moisture is absorbed and stray voltages can affect reliability.

Q: What recommendations would you make to increase the ability to remove residues using saponifier #1?

A: Increase the concentration and maintain a wash temperature of 60 degrees C. I recommend using lower pressures in the prewash and wash sections, at a slower belt speed, and evaluation of saponifier #2. Prewash and wash pressures should be set at 35 and 30 psi (top and bottom respectively) and belt speeds set at 3.0 ft/min.

Conclusions

A high I/O microBGA, with a low standoff under standard aqueous (water only) cleaning techniques produced a product that left visual residues and ionic levels well above cleanliness limits for good electrical performance. The boards cleaned with the water-soluble paste and saponifier #1 showed a high level of chloride, bromide, WOA residues and visible flux residues. Saponifier #2 showed great visual and ionic cleaning performance and offered less risk of electromigration problems. Further testing, using surface insulation resistance (SIR), is currently planned after process optimization to complete the study.

---

Terry Munson is with Contamination Studies Laboratory (CSL), Kokomo, IN; [phone]; e-mail: Residuguru @aol.com; Web page: www.residues.com.

http://www.circuitsassembly.com

Copyright [copyright] 2001 CMP Media LLC