Adhesion Enhancement for Improved MSL
10/01/2008
BY OLAF KURTZ, CHRISTIAN WUNDERLICH, ROBERT RÜTHER, JÜRGEN BARTHELMES, Atotech Deutschland GmbH, DIN-GHEE NEOH, Atotech S.E.A. Pte Ltd
With the development of environmentally friendly ICs at reduced thickness, the manufacturing of thermally reliable products becomes more demanding. To achieve an equivalent reliability standard for state-of-the-art ICs like quad flat no-lead (QFNs), expressed in terms of achievable moisture sensitivity level (MSL), new technologies have to be introduced. One such technology, a patented process characterized by an extended bath life, which provides high metal holding capability and contains an effective smut removal agent for use with the silicon containing C7025 base material, was developed to address this. The resultant surface topography ensures enhanced bonding between the leadframes and the mold compound used in IC assembly. This process provides an improvement in achievable MSL for all package types.
Why Leadframes Need Adhesion Enhancements
Since the 2006 EU legislation imposed lead-free production for most electrical and electronic equipment, the IC/leadframe and connector industries have been searching for lead-free solderable alternatives. A consequence of this change is the need for higher reflow temperatures due to higher soldering melting points, while the miniaturization trend results in package designs that generate more heat per area. On the basis of the IPC/JEDEC J-STD-20, the electronic industry has therefore defined a standard for the MSL classification.1
To keep cracking from occurring when the device is heated, several substrate treatment processes have been developed to provide a firm bond between the copper alloy and polymeric material surfaces. Generally, etchants are more effective on pure copper substrate, providing better adhesion characteristics compared with copper alloys. When peroxide-based etchants are used for C194 copper alloy (containing 2-3% Fe), dissolved iron will rapidly degrade the hydrogen peroxide, dramatically (and irreversibly) reducing the product life.2
Alloy C7025 (containing <1% Si), also creates further problems due to Si smut residues that may contaminate the silver surface after processing, resulting in wire bond failure and reduced adhesion. It is imperative to remove this smut using a post-treatment operation, although current post-dip technology to clean C7025 surfaces causes loss to surface roughness with the ensuing adhesion loss.
Adhesion Enhancing Process
The principle treatment step in the leadframe adhesion enhancement process* has the capability of dissolving high levels of copper. In the activator step, the exposed copper is treated with a preparation additive that provides the desired uniform brown, organo-metallic coating, resulting in a surface topography that ensures enhanced bonding between the leadframes and the mold compound used in IC assembly.
During the copper etching process, a soluble adhesion layer is formed, attaining a maximum thickness when equilibrium exists between formation and dissolution.3,4 The combination of increasing roughness and the formation of a Cu(II)-organic compound results in a color change to the leadframe as etching time progresses.
The proprietary organic additive forms a porous layer on the substrate surface. The density of this layer controls the diffusion of etchant to the surface, directly influencing the resulting etched microstructure.5,6,7 The additive provides the preferred surface topography by controlling inter-granular etching (IGE) of the copper crystals grain boundaries.
Bonding to the mold compound is believed to occur due to a combination of mechanical and chemical adhesion. Mechanical interlocking is achieved due to a surface area increase, while chemical interaction occurs due to polar and covalent bonding between the molding compound and the organo-metallic coating. The physical and chemical characteristics of this surface result in excellent MSL performance of IC packages.
Process Sequence
The sequence begins with the pre-treatment operation consisting of a mild etch and an alkaline cleaner, the purpose of which is to remove any difficult surface residues from preceding processes, e.g. stamping. Prior to entering the main etching stage, leadframes are immersed into the activator to minimize drag-in of contamination and providing a light surface oxidation.
The leadframe then enters the acidic micro-etch stage, which provides controlled etching and dramatically increased surface area morphology due to the presence of organic additives. Etch depths depend on application requirements and may range between 0.8 and 1.6??m.
 Figure 1. Atomic force microscope (AFM) images show the stepwise increase of leadframe surface roughness. |
The sequence is finalized using the post-dip treatment, ensuring satisfactory cleaning of organic contamination from the silver surface that may later lead to deterioration of gold wire bondability.
Roughening Study
Figure 1 illustrates the stepwise increase of leadframe surface roughness. The table below provides corresponding relative surface area increase (RSAI) data. Note the difference between the 3D surface area and its 2D, down-projected area and Ra (Average surface roughness) values. The preparation additive also incorporates an effective smut removal additive for the successful treatment of silicon containing C7025 leadframe material. All leadframe materials can be treated using the same process sequence and products. Figure 2 shows the inter-granular etching on the copper surface.
 Figure 2. FIB cross-section showing the roughened copper surface by inter-granular etching (x5000). |
Etch rate and the RSAI increase measurements after preparation additive treatment have been carried out for the main leadframe base materials, i.e. C194 (2.4% Fe, 0.03% P, 0.1% Zn, balance copper), C7025 (3.0% Ni, 0.65% Si, 0.15% Mg, balance copper) and the fully hardened MF202 (2.0% Sn, 0.2% Ni, copper balance).For all three base materials, maximum RSAI is achieved at an etch depth of 0.8 ??? 1.6μm (dependent on material type). C7025 has the lowest maximum roughness at a low etch depth of ~0.8 μm, while MF202 exhibits a lower initial RSAI during etching but can achieve high roughness values, if etching proceeds. This additive can treat all fully hardened Cu alloy material, including MF202. A common etch rate of 1.2μm/min is observed for all the tested base materials. A surface topography study for these three copper-alloy base materials highlights marked differences (Figure 3).
 Figure 3. Different appearance of the three different copper base materials in SEM images (x5000). |
To compliment this roughness study, the influence on the silver surface was investigated and proven that no attack occurs to the ring or spot silver surface. The post-dip solution, developed for use in reel-to-reel and strip-to-strip applications, maximizes the cleaning of the silver surface after the preparation procedure, ensuring perfect bondability of the silver during the gold wire bonding stage. The post-treatment has no negative impact on the copper roughness.
Bath Life Extension
This mold preparation additive was developed to incorporate an iron complexant to maximize bath lifetime. An extensive study, using both laboratory and production conditions, investigated process decomposition at varying complexing agent levels.
 Figure 4. The curves show the dramatic effectiveness of complexing agent presence to extend the bath life, as a function of Part B decomposition rate with time. |
Figure 4 shows the stabilizing influence of the complexing agent against Fenton’s reactions for the preparation additive bath with obvious extension to process lifetime.
The Button Shear Test
The button shear test (BST) measures the interfacial bond strength between the molding compound and leadframe. Extensive studies on the adhesion strength were performed to correlate the influence of certain process factors to the final reliability result. Test samples of the 5?? common molding compounds were shear-tested at both room temperature and 260??C. The results are summarized in Table 1 and show adhesion improvement for all molding compounds after treatment.
 Table 1. Summary of the BST results for different molding compounds tested at RT and 260??C. Note: Unit = N/mm2 |
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MSL Results and Improvement
The test results provide data that highlight a general MSL improvement of at least one level, comparing with and without mold preparation additive treatment. MSL1 classification can be achieved for state-of-the-art IC packages like QFN, TSOP, and QFP (Figure 5).
 Figure 5. Silver ring plated QFN package with treatment showing MSL1 at 260??C (left) in comparison to conventional silver ring plating (right) showing massive cracking. |
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Gold Wire Bonding
Gold is favored for wire bond processes due to its physical and chemical properties, characterized by low electrical resistance, high ductility, and excellent corrosion resistance. Gold wire investigations, based on the MIL-STD-883F standard, show that by using the post dip additive and process, bondability to the silver surface can be significantly improved. As an example, two different types of packages have been tested, SOIC and the ETQFP. All treated samples show very good results concerning the pull forces (Table 2).
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Conclusion
A patented process has been described for the enhancement of the adhesion between molding compound and the copper alloy substrate. The leadframe is treated with an inter-granular etch solution containing proprietary additives, resulting in the formation of an organometallic layer on top of the roughened alloy surface. During the subsequent molding process, mechanical interlocking between resin and the copper alloy substrate is established. Furthermore, due to chemical interaction, an increase in the thermal stability of the IC at elevated temperatures can be observed. The resultant surface topography ensures enhanced bonding between the leadframes and the mold compound used in IC assembly. The additive contains an effective smut removal agent for use with the silicon containing C7025 base material. These improvements to the mechanical and chemical properties of the treated surface for all common base materials ensure excellent MSL performance of IC packages.
* MoldPrep HMC from Atotech
References
- IPC / JEDEC J-STD-020C, Moisture/Reflow Sensitivity Classification for Nonhermetic Solid State Surface Mount Devices, July 2004.
- Walling, Cheves “Fenton’s Reagent Revisited”, in Accts of Chem. Research, vol. 8, pp. 125-131 (1975).
- J. Barthelmes, C. Wunderlich, D.-G. Neoh The successful application of leadframe roughening for MSL level improvement, 15th ASEMEP National Technical Symposium, 2005, Philippines.
- J. Barthelmes, C. Wunderlich, D.-G. Neoh, S.W. Kok, A reengineered and versatile leadframe roughening process for MSL improvement, IEMT 2006, Malaysia.
- C. Sparing, internal report, Atotech Deutschland GmbH.
- M. E. Biggin, A. A. Gewirth, Journal of the Electrochemical Society 148 (5) C339-C347 (2001)
- J.T. Huneke, Die attach adhesion on leadframes treated with antioxidants, IEEE/CPMT Conference 1997, Singapore.
- DOD, Test Method Standard Microcircuits, MIL-STD-883F, 2004.
OLAF KURTZ, Ph.D.;CHRISTIAN WUNDERLICH; ROBERT RÜTHER,Ph.D. and JÜRGEN BARTHELMES, Ph.D. may be contacted at Atotech Deutschland GmbH, Erasmusstra??e 20, D-10553 Berlin; +49/30 34985 447 E-mail: [email protected] DIN-GHEE NEOH, may be contacted at Atotech S.E.A. Pte Ltd, 20 Tuas West Road Singapore 638379, email: [email protected]