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FR-4 has proved to be a reliable and less expensive alternative to ceramic.

Numerous companies in automotive, military, commercial and other electronics sectors have migrated from using ceramic substrates to PCBs. In some instances the ceramic substrate was overengineered and the PCB was capable of handling the heat dissipation and temperature exposure of the application. In some cases, two substrates exist within the same module, one PCB and one ceramic.

Chip-on-board, once the act of wire bonding from an IC to the PCB with small diameter aluminum wire, now includes large diameter wire for bare die-to-substrate and interconnecting the housing leadframe to the substrate (Figure 1). Wire diameters from 0.005" to 0.020" are included in large wire sizes.

The most common circuit board type today is FR-4. FR-4 substrates are significantly less expensive than ceramic substrates. There are more FR-4 vendors making boards than there are ceramic substrate suppliers. FR-5 is an alternate material that is suitable for wire bond applications. Compared with FR-4, FR-5 has a higher flexural strength at elevated temperatures and a higher maximum operating temperature. Fine-line resolution typically achieved in ceramics is available with organics, and 0.002" lines and spaces are not uncommon (Figure 2).

Switching from a stable ceramic substrate to an organic substrate is not without challenges but the cost savings more than make up for the effort. Construction of the base material is basically the same as for a multilayer PCB. Glass sheets impregnated with epoxy resin are layered on top of other sheets and laminated together under pressure and heat, typically in a vacuum, to make the final substrate. Compared to co-fired ceramic, the resulting substrate is not that rigid, and ultrasonics may disperse randomly through the PCB structure when applied. This may cause inconsistent bonding quality.

Bonding directly to the base copper of the PCB is typically not possible as the surface becomes heavily oxidized when manufacturing the board. For large wire bonding, put Al-clad copper bond pads on these bond locations or use plating. Since the PCB will also have soldered components, a suitable, multifunctional plating is recommended. Electroless nickel/immersion gold (ENIG) is one of the more popular platings used for aluminum wire bonding. The thickness of ENIG deposit, as specified by IPC-4552, is 2 microinches minimum at -4 sigma from the process mean for the gold and 120 to 240 microinches for the nickel. A typical value of 3 to 5 microinches of gold should be expected. The phosphorus content of the nickel ranges from 4 to 9% for "mid phos" baths to 10 to 14% for "high phos" baths. Mid phos baths are more common. The function of the gold deposit is sacrificial, used only to prevent the nickel surface from passivating, thus causing both solderability and wire bonding issues. Aluminum wire bonding is made to the nickel, not to the gold on the surface.

Immersion silver (ImAg) is also capable of aluminum wire bonding (Figure 3). However, the bond pad needs to be encapsulated to prevent a metallurgical change due to air exposure that might result in long-term bond failure.

When deciding which FR type to use for wire bonding, consider the following factors. Choose a glass transition temperature (Tg) that would be safely above the highest operating temperature. Tg has a direct impact on the PCB z-axis expansion. Above the Tg of a material, the expansion rate increases at an exponential rate. Evaluate the Tg of the material by DMA (dynamic mechanical analyzer) as opposed to DSC (differential scanning calorimeter) as it is the mechanical characteristics of the material that have a major impact on wire bonding, rather than the heat capacity of the material.

The Tg of FR-4 and FR-5 is similar. There are three separate temperature ranges for these boards: 110°, 150° and 175°C. Use of the lower Tg materials in the U.S. is on the wane, as they are being replaced by the 150°C minimum. Laminate materials are specified in IPC-4101 with each material set assigned a "slash sheet" designation. FR-4 is currently specified by /21, /24 and /26, which correlate to Tg minimums of 110°, 150° and 175°C, respectively.

The rapid increase in z-expansion as a function of exceeding the material's Tg may result in false touchdowns, cutting problems, over-bonding and faster aluminum build-up on the bond tool, in addition to other problems. Be mindful of having the substrate attachment to the backplate or housing exiting the curing oven and being transported to the wire bonder while still hot or warm.

Another critical variable to consider is the camber (also known as warp and twist). Most board manufacturers can meet the industry standard of 0.0075" per inch. The bigger the board, the larger the inherent camber. FR-4 boards have a memory, so regardless of whether an internal process makes the boards flatter, after a temperature excursion the board may likely re-form to the natural camber.

When working with FR-4 in wire bonding applications, attach the board to a rigid surface. Ensure the fixturing so that the module cannot move in any of the axes during wire bonding as this may cause sliding and inhibit the attachment of wire to surface.

The most common wire type for these applications is 99.99% aluminum. Wire specifications have changed over time.¹ Typically this change has been toward a more ductile ("softer") wire. Aluminum wire of 0.10" diameter is common in FR-4 applications. A typical elongation could be 10 to 18% with a tensile strength of 350 to 450 g. Years ago, the tensile would have been 450 g minimum.

Visteon has been involved with the conversion of ceramic substrates to FR-4 boards for automotive applications. These conversions are primarily for cost-reduction reasons. The company has used ImAg on bond pads with 0.032" thick FR-4. Performance and durability of these wire-bonded automotive modules has been excellent.²

New Trends

According to Casey Cooper, "Recent R&D efforts at STI have resulted in an emerging assembly and manufacturing technology coined Imbedded Component/Die Technology."³ This assembly technology enables reduced size and weight and increased functionality and reliability. The size and weight shrink is due primarily to the exclusive use of bare die, eliminating external component packaging and high-volume, dense solder interconnects. To meet the varying requirements of the electrical system, two wire diameters are often used to eliminate conductor burnout. Overall assembly weight is reduced by using a high-temp FR-4 substrate with electroplated nickel/gold over copper pads. STI found that the tradeoff of flexible wire bonds (small and large diameter) over hard solder joints increases an assembly's robustness and reliability in harsh environments such as exposure to vibration, shock and varying thermal environments (CTE movement). Figure 4 shows 0.005" wires on a bare die mounted on a PCB.

Large diameter wire bonding is feasible in board applications. This will continue as PCB technology advances. The migration from ceramic- to organic-based substrates has begun in a few industries and will continue to expand into others. The Pb-free process does not require any post-cleaning steps and is done at room temperature. Wire bonding interconnection has proved itself in high-reliability segments since the 1970s and continues to make inroads into new markets.

References

1. Frank Grosso, SPM.

2. Al Schaller, Visteon (Van Buren, MI).

3. Casey Cooper, STI Electronics (Madison, AL).

Mike McKeown is sales manager at Orthodyne Electronics (orthodyne.com); mike.mckeown@orthodyne.com. Gerard O'Brien is an engineer at Photocircuits (photocircuits.com); gobrien@photocircuits.com.