Traditional boundaries of backend process and SMT assembly have become diffused.

Within mainstream surface mount assembly, bare die wire bonding directly to the PCB substrate and adjacent to other soldered components has long been practiced for low end to high performance assemblies. The backend IC industry, with its decades of understanding wire bonding metallurgy, finds itself acquiring and adapting SMT know-how to the field of packaging. The surface mount industry, however, may find it relatively harder to deal with the complexities of wire bonding. This article reviews metalization options and interactions for COB, while considering needs of the bonding process and soldering.

Apart from size and footprint advantages, organic-based packaging offers a significant spinoff: It is a platform readily adaptable by the IC backend toward attaching multiple dies and other active and passive components on a substrate within an IC (BGA or CSP). After overmolding, the resulting assembly creates a highly functional device. Systems-in-package (SiPs) and multichip modules are two examples.

Thus, after decades of packaging – typically – a single die on a metal lead frame, the face of IC backend is being permanently transformed and acquiring the flavor of a PCB assembly-like process, albeit one involving dies and surface mount all within the package itself. Other than the final overmold and ball attach, if applicable, the process replicates the COB-SMT concept as deployed for mainstream surface mount assembly.

COB metalization basics. In the most common COB layout (Figure 1), bond finger pads are arranged in an array around the die (fanout only). P (power) and G (ground) rings may be included within the signal I/O bond finger ring. Several P and G rings may be permitted; however, when present, rings must be covered by solder mask, except at the bonding sites. This protects the metalization and prevents inadvertent shorts.


Bond finger pads must respect PCB design rules for line spacing and width. Solder mask openings in the bond finger are used only to permit bonding in open areas. (Figure 1, bottom left).

Die placement area metalization. If electrical conductivity to die is required, then die placement area metalization is needed. Figure 2 shows a typical example of pad sizing, wire lengths and layout (based on a pocket dictionary product example) for a die of approximately 9.5 x 5 mm with an I/O count of 184. The pad length enables a one-time re-bond (rework). For reliability reasons, re-bonds are never performed over previously bonded spots on the pads.¹


In high reliability, industrial applications, or areas such as packaging, this metalization must provide a barrier to copper migration into silicon die. Gold over nickel is used for high-end applications, with nickel providing a barrier against copper migration. For the low-end, nickel (sans gold) is adequate for low-cost applications.

As a rule, dies for power applications are not advised for COB because of the significant CTE mismatch between the silicon die and organic substrate.¹ Overmolded assemblies such as SiPs or MCMs are more robust and tolerant of this aspect. Placement metalization may be designed to improve thermal performance if needed for a specific situation. This would take the form of vias (copper filled) in the placement metalization connected to the bottom or innerlayers.

If electrical conductivity to the die itself is not required, placement areas with solder mask become feasible. Significant variables to consider are the compatibility of the die bonding material to the mask, the mask cure condition, absorbed moisture and contamination – all of which can affect surface activation energy and hence die-substrate adhesion. With solder mask, placement coplanarity becomes an important variable: A few mils of die tilt can lead to mis-bonds during wire bonding process.¹

Bonding variables fishbone diagram. Figure 3 shows the variables fishbone tree. The process itself is far more complex and sensitive to any encountered in mainstream SMT. When variables for both the SMT process and COB are combined, the resulting fishbone diagram appears formidable. Solder joint reliability is to surface mount as bond reliability is to the bonding process – the latter co-related to bond pull strength. As Figure 3 shows, the metals and resulting bond metallurgy play a significant role in attaining this important parameter. Understanding both the metallurgical interactions in COB and the soldering metallurgy is essential to process development and failure analysis.


Bonding Wires

Gold and aluminum bonding wires, the backbone of IC wire bonding for decades, continue to be default choices. Materials such as copper wire and silver surface metalization are also coming into focus, although use is not as prevalent in COB. Among the important mechanical considerations influencing bonding wire selections are tensile properties, elongation (EL) and break load (BL).²

Electrical properties are application dependent; however, each material produces different metallurgical interactions with the die pad metalization and substrate metalization. Because substrate metalization interacts uniquely with different solder alloys, it is therefore an important consideration in balancing the needs of wire bonding and soldering. Table 1 compares bulk properties of various materials.



Gold bonding wire. Gold ductility provides the flexibility needed for good loop formation. Gold has excellent electrical and thermal properties, and its inert state (gold is a noble metal) makes it well suited to the bonding process. Gold wire is widely used in both ball and wedge bonding applications, producing reliable bonds to aluminum and gold surface metalization.

Needs for flexibility, loop retention and bondability must be balanced. Pure gold wire is too soft and is usually stabilized with dopants such as beryllium (around 5-10 ppm) or copper (around 10-30 ppm). Based on purity, gold wire is classified into various ranges from 99% (2N gold) to 99.99 % (4N gold). Beryllium-doped gold wire is 10-20% stronger than copper-doped and hence better suited for high-stress applications such as high-speed automated bonders. Gold wire is typically supplied in an annealed condition to prevent unwanted breakoffs during initial bond formation.

Table 2 compares properties of 4N versus 2N gold as developed by one supplier for “ultra stiff” gold wire bonding applications.³ It is interesting to note the differences in the minor and the major dopant levels (none in the case of 4N wire).


Surface cleanliness and contamination are critical to bond strength. The bonding metalization surface finish is critical to prevent bond tool capillary clogging. Some tradeoffs with gold bonding wire are:

• Cost.
• Requires expensive bonding tools – ceramic for ball bonding, titanium carbide for wedge bonding.
• Requires high-temperature bonding, a possible source of die stress.
• Limitations in heating the (organic) substrate metalization.

Although gold wire bonds can be made without substrate or wire heating, reliable gold bonding requires heating the bonding metalization and wire to a high temp. Ball bonding requires temperatures of 220-250°C, while reliable gold wedge bonding can be accomplished at around 150°C.

Given the form factor, high thermal conductivity and uniform thermal mass, heating a metal lead frame or die pad (for example, preplated copper lead frames as used in IC manufacturing) is relatively easier compared to organic substrates. Organic PCB substrate variables – lower thermal conductivity, complex thermal mass distribution, size, CTE, whether the PCB is populated or unpopulated – make it less than ideal to create localized heating at the substrate pads. The process may slow throughput or lead to higher stress conditions, marginalizing the overall assembly reliability.

Al-Si bonding wire. Small diameter aluminum wire is often used for wedge bonding and offers relatively good fatigue resistance. As is the case with gold, pure aluminum is too soft to draw into small wires, so it is alloyed with about 1% silicon to provide the desired properties of load and elongation.

Lightweight Al-Si wire is very reliable but is much less expensive than gold. It is used extensively with low-cost wedge bonding tools (tungsten carbide) and, because Al-bonding can be performed at room temperature, it is well suited to organic substrates and therefore an ideal default choice in most low-to-mid-cost COB applications.

As Figure 4 shows, 1% silicon exceeds the solid solubility limits of silicon in aluminum at room temperature. Silicon precipitation can cause stress and possible wire fracture and is one of the failure mechanisms associated with heel cracks.


Small diameter Al-Si wires are usually heat-treated (partially annealed) to disperse silicon uniformly. Large diameter wires are heat-treated before and after final drawing.

Al-Mg bonding wire. Aluminum alloying with magnesium is an alternate to alloying with silicon and offers certain advantages. Figure 5 shows the Al-Mg phase diagram. Magnesium solid solubility in aluminum is better than the 1% maximum of silicon dissolved in aluminum.



Aluminum with 1% magnesium can be drawn into fine wire with similar strength as aluminum with 1% silicon. Al-Mg gives satisfactory bonding and is superior to aluminum with 1% silicon in fatigue failures. It also shows superior ultimate strength after high temperature exposure. Despite this, use has remained less prevalent compared to Al-Si, which has become widely accepted.

Copper bonding wire. Copper ball bonding has seen more recent use, and in particular, copper ribbons have received considerable attention. Copper is economical, has excellent heat and electrical conduction – hence smaller wires possible – and is resistant to sweep during plastic encapsulation.

Copper to copper bonding is possible, but the major issue remains bondability. Since copper oxidizes readily, bonding in inert atmosphere is needed, which somewhat negates the cost advantages of copper bond wire.

Cu-Al intermetallic growth rate is lower than Au-Al, and provided the initial bondability issue is addressed via inert gas, copper offers better reliability than gold wires to aluminum pads with 0.4-4.0 mil wire diameters. However, copper is harder than silver and aluminum (Table 1) and risks die cratering or pad metalization damage – a harder pad metalization is required for copper wire bonding. Currently, copper use is limited to mainly high-end use (CSPs, QFNs); use in COB applications is limited.

Metallurgical Systems

Gold wire-gold plated pads. A mono metal system, the Au-Au bond is extremely reliable. It is not subject to interface corrosion; there is no intermetallic formation and no bond degradation. Even a marginal Au-Au bond performs well despite time and temperature. As mentioned, gold is best bonded with heat. Cold ultrasonic Au-Au bonding is possible but less reliable. Thermosonic bonding is preferred and most common; however, thermocompression bonding is possible. Bonding is highly affected by surface contamination. Au-Au bonding is used in high-end COB applications and within the IC industry for applications such as SiP and MCMs. Plasma etching is extensively used to improve surface activation in high-end processes. Gold plating thickness for the bond pad metalization is target application (reliability) dependent. For high reliability bonding, an ultra pure, soft gold (hardness 60-80 knoop) in the range of 30-100 microinches is typical.

Soldering vs. bonding on gold pads. Gold offers excellent coplanarity, fine features and high-density circuits; it is essential in high-frequency applications. Gold can withstand multiple reflows, has good corrosion protection and excellent wetting. The main issues with gold are cost and a hard-to-manage plating process.

ENIG. Many industry segments have shied from ENIG (electroless nickel immersion gold) as a surface metalization for fear of “black pad” defect. With the prevalent use of the ENP (electroless nickel phosphor) process, which co-deposits phosphor in the nickel plating, the impact of phosphor content in the ENP plating on bond reliability and solderability has been the subject of several studies.⁴ Traditionally, 6-8% phosphor content has been used, but some recent studies show otherwise – up to 10-12 % phosphor content producing a more favorable, corrosion-resistant nickel surface morphology, an important variable, making it less prone to attack by the subsequent gold plating layer or the eventual soldering process.⁴

In the author’s experience and from a metallurgical standpoint, a controlled⁵ and specified ENIG process offers an excellent surface finish for soldering (with SnPb and Pb-free solders) and wire bonding, but the ideal thickness requirements for the gold film are quite different for the two methods. For soldering purposes, gold must be a dense (not porous), thin film, whereas for bonding, it should be thick, pure “soft” gold. The general structure favored for soldering applications is a “dense” gold of around 2-4 microinches over an underlying nickel layer of 200-240 microinches.^4,5,6,7

MIL-QQ-N-290A specifies 200 microinches of nickel between the copper and gold layer as a copper diffusion barrier. IPC-4552 specifies a minimum gold thickness of 0.05 µm [2 microinches] for (statistical) process variability. It also cautions on possible appearance of black pad when immersion gold thickness approaches 0.25 µm [10 microinches]. The gold layer acts as a sacrificial film in protecting the underlying metal, and needs to be sufficiently thin to be consumed by the tin during soldering, with the resulting AuSn IMCs dispersing into the bulk solder fillet. The bond is thus formed to next layer – nickel or copper, as the case may be – which must be active, lest it defeat the gold’s role. Soldering to thicker gold films meant for bonding applications results in a large percentage of AuSn IMC precipitating at the solder joint to pad interface, at the risk of solder joint embrittlement and low cycle life.

For COB-SMT, then, the ideal practice with gold would be to follow the structure desirable for soldering combined with selectively plated “thick” gold at the bonding sites. This process is costly and used mainly in high-end COB applications. If the product positioning does not justify it, a compromise has to be made, in which case the thin gold (favoring soldering) is chosen; it provides a product life commensurate with product expectation.

DIG. Direct immersion gold is a relatively new process in which gold is plated directly over copper.6 The gold film properties (thickness, porosity, morphology, etc.) apply to DIG as much as to ENIG, however, during soldering as the tin bonds to the copper, forming SnCu IMC, which has been documented to have a higher growth rate than the SnNi IMC layer.⁸ Hence, a lower reliability may be expected compared to ENIG, but it may well meet the lifecycle of many products. DIG field experience is limited in COB applications, but it “appears” well suited and cost-effective.

Gold wire-aluminum die pads. Au-Al is very common in wire bonding for COB applications. However, there are reliability issues over time. It is easily subject to Au-Al IMC layers and Kirkendahl voids. IMC formation accelerates with operating time and temperature and five IMC layers are formed as listed below (Figure 6).

• Au5Al2 (tan color) (some studies suggest Au8Al3).
• Au4Al (tan color).
• Au2Al (metallic gray color).
• AuAl (white color).
• AuA12 (deep purple color).

It is believed that initially AuAl2 forms at the Au-Al interface, then transforms to other IMCs with time and temperature.

Au-Al intermetallic growth. In a controlled ball bond process, Au-Al IMC growth shows relatively planar morphology. IMC initial growth rate is believed to be parabolic, settling to about 3-4 µm over time. At high temperatures (175°C) the aluminum pad converts to IMC; Au5Al2 (gold side) and Au4Al (pad side) predominate in the IMC layer. Au4Al may also grow by consuming Au5Al2 (Figure 6). Au4Al is susceptible to corrosion by epoxy molding compounds used in COB and may also oxidize.

Gold wire-copper PCB pads. Gold bonding to copper pads is possible but rarely used in COB. Three IMC phases are formed: Cu3Au, AuCu and Au3Cu. IMC formation decreases bond strength at high temperature (200 to 325°C). Kirkendahl voiding can occur due to copper migration. Bond strength degradation is dependent on micro-structure, bond quality and impurities in the copper. Cleanliness is extremely important for bondability and reliability when bonding to copper. Use of inert gas (argon) shielding improves bondability and reliability, prevents copper oxidation and is needed for curing polymer die attach material.

Soldering aspects. While copper with OSP would be a good choice for soldering low-end products, OSP coated pads are not considered usable “as is” for wire bonding. The alternatives of copper OSP pads for soldering and selectively plated (gold, silver or nickel) pads for wire bonding, while feasible, again negate the cost advantage and are hard to justify.

Gold wire-silver pads. The Au-Ag is very reliable for long terms at high temperature. Au-Ag does not form intermetallic compounds. Gold wire bonds to silver lead frames or silver plated pads have been successfully used in high-volume production for years. Silver bondability issues are caused by contaminants like sulfur that tarnish the silver plating. Au-Ag high temperature thermosonic bonding is performed at about 250°C and improves bondability by displacing the tin sulphide films. Use of silver is currently not prevalent in COB applications for cost reasons and thermosonic bonding requirement.

Soldering aspects. The immersion silver process is similar to immersion tin, except it uses electroless silver deposits. Immersion silver is being promoted as an alternative to Gold (due to black pad fears with ENIG) – and can be adopted into COB applications. However, silver metalization on solder pads is not free of solder joint reliability issues. Microporosity at the Ag-Cu interface occurring from the silver-plating process (corrosion of the copper surface) has been reported and deemed a reliability threat to solder joints. Silver solderability can easily degrade in contact with sulphur compounds. Thickness depends on two types of chemistry for immersion silver: “thin” silver (min. 0.05 µm ) and “thick” silver (min. 0.12 µm). These thicknesses are expected to guarantee a minimum one-year shelf life. Upper thickness limits for both types of immersion silver were not established in the initial release of IPC-4553. However, immersion silver is not an allowable finish for Class 3. Another reason why silver is being promoted is to eliminate tin whiskers. Costs are similar or marginally more than immersion tin. Silver use is relatively new and not common in the high-volume, cost-sensitive COB segment. Further field experience and volumes are needed to characterize long-term impacts.


Aluminum wire-aluminum die pads. Al-Al bonding is an extremely reliable system. Being a mono metal bond, it is neither prone to IMC formation nor corrosion. Al-Al bonding is best performed ultrasonically. Al-Al thermocompression bonding is possible by high deformation. Al-Al wedge bonding is predominant in COB applications, the mainstay of the low-cost, disposable product range and also often used in midrange products.¹

Aluminum wire-nickel PCB pads. Al-Ni bonds are typically made with large (greater than 75 mm) diameter wires. Large wires bonds are less prone to Kirkendahl voids and galvanic corrosion. Nickel bonds are more reliable than Al-Ag or Al-Au. Electroless nickel on pads from boride or sulfamate base systems give reliable bonds. Aluminum to nickel pad bonding remains very popular in COB applications, low-cost yet reliable.

Bondability is affected by nickel surface oxidation, and measures to improve nickel pad bondability include the (short) time between plating and bonding; PCB storage in an inert atmosphere; and chemical cleaning before bonding.

Soldering with nickel pads. Aforementioned measures to improve bondability also apply to improve solderability. Electroless nickel coated components have a short shelf life (fewer than 24 hr., to meet MIL-893C solderability standards) unless protected over by gold. Issues also relate to the phosphorus content in EnP plating. Storage conditions are important, as is first in, first out). Use within six months from date of manufacture remains a related concern with nickel.

Copper bond wire-aluminum die pads. The Cu-Al system is prone to various IMC failures similar to Au-Al systems. IMC growth in Cu-Al is lower than in Au-Al systems and no Kirkendahl voiding is seen in Cu -Al IMC. Brittle CuAl₂ in IMC layer lowers shear strength at 150-200°C; at 300-500°C, excessive IMC growth significantly reduces bond strength. Cu-Al IMC is impacted by atmospheric composition. Presence of O₂ creates copper oxides, which inhibit bondability by growth of void-like grooves under the bonds. Chlorine contamination and H2O (moisture) can corrode the aluminum and Cu-Al IMC layer.

Metalization and Product Positioning

Figures 7 to 12 show several diverse applications of COB-SMT. Figure 7 is an assembly from a handheld toy fan with motion activated flashing lights. It uses a single-sided BT resin PCB with OSP over copper pads and nickel over copper bond pad metalization. At the next price point, Figure 9 shows the COB in a PDA using a multilayer FR-4 PCB with uniform nickel metalization at both soldering and wire bond sites with AlSi wire bonding process. Figure 10 is the display and control module from a microwave oven and Figure 11 shows the main and remote control assemblies from a mini-disk player. Both products use ENIG pads for both soldering and wire bonding. Figure 12 is the assembly from a high-end heart rate monitor using ENIG at the solder sites and selectively plated soft pure gold at the COB sites. The common factor in each of the shown products is COB, yet metalization choices are dictated by product positioning, life, reliability expectation and price point. The cost-sensitive low end uses metalization that leans in favor of the soldering process.


Conclusion

Wire bonding is not out of steam, given improvements in the latest generation of bonders.⁵ The metalization choice can help provide the right balance between wire bond reliability, soldering yields, and solder joint reliability. At the low-to-mid cost spectrum, the metalization tends to lean toward alignment with the soldering process. Despite this compromise, the wire bonding process for this product class proves sufficiently robust for the application.

The required metalization for reliable wire bonding is not necessarily the same as that needed for high quality solder joints. ENIG – or its derivative, ENEPIC – provided the nickel and gold plating control is carefully maintained^4,6,7 when combined with selective pure thick gold on the wire bond sites, provides the highest reliability for both soldering and wire bonding but also incurs the highest cost.

At the next lower level, ENIG is an option for both soldering and wire bonding without selective plating on the wire bonding sites. ENIG metalization yields an excellent solder joint quality and while not “the” best for wire bonding, proves adequate for midrange product needs.

Where cost sensitivity is acute, either OSP over copper for solderability with selective nickel or gold plating on the bond sites, or simply a uniform nickel as a common soldering and bond site finish together with aluminum wedge bonding, provides the lowest-cost alternative.

References

1. Mukul Luthra, “Process Challenges And Solutions for Embedding Chip On Board Into Mainstream SMT,” SMTA International Proceedings, September 2003.
2. American Society for Testing & Materials, F72-06, Standard Specification for Gold Wire for Semiconductor Lead Bonding.
3. International Directory of Solder Reflow Thermal Processing Equipment, Chip Scale Review, April/May 2007.
4. Kuldip Johal et al, “Impacts of Bulk Phosphorous Content of Electroless Nickel Layers to Solder Joint Integrity and Their Use As Gold-Aluminum Wire Bond Surfaces,” SMTA Journal, April-June 2004.
5. Ron Iscoff, “Are Wire Bonders Running Out of Steam? How This Essential IC Assembly Tool is Keeping Pace,” Chip Scale Review, March 2007.
6. Shigeo Hashimoto, et al, “Electroless Gold Plating for Printed Circuit Boards,” IPC Apex Proceedings, February 2007.
7. Kuldip Johal and Gerry Brewer, “Are You in Control of Your Electroless Nickel / Immersion Gold Process,” SMTA International Proceedings, September 2001.
8. R.J. Klein Wassink, Soldering in Electronics, ISBN 090115024-X, 1984.

Ed.: This article was first published at SMTA International in October 2007, and is used with permission.

Mukul Luthra is business director and founder of Waterfall Technologies (waterfalltech.com); mukul@waterfalltech.com.