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Posted on 01 March 2019

The Challenge of Packaging Small Power Devices

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Better Electrical Performance, Lower Cost, High Reliability

The demand for smaller and thinner devices is dictating the development of packaging and interconnecting techniques for discrete power semiconductors. This trend is driven by requirements of space-critical end applications, and enabled by ongoing substantial improvements in silicon efficiency.

By Siegbert Haumann, Christoph Luechinger, Orthodyne Electronics Corporation

 

Up until now, fine wire ball bonding and Copper strap attachment have been the main interconnect techniques used in power packages smaller than the TO-252 (DPAK). However, increased requirements for better electrical performance, higher reliability and especially lower overall cost, now expose weaknesses in each of these existing techniques. Large Aluminum wire bonding, the most commonly used interconnect technique in DPAK and larger size power devices, was so far not a viable alternative due to the package size requirements. However the technique’s proven strengths, reliability, flexibility and low overall cost are very much desired for producing modern small power packages in an economical way.

The recently introduced large Aluminum ribbon bonding process (PowerRibbon™), developed by Orthodyne Electronics, overcomes the package size restrictions and allows the application of the strengths and benefits of the large Aluminum wire bonding process in smaller power packages. It adds the additional benefits of a wide process window and higher productivity. All in all, PowerRibbon™ bonding offers a near perfect fit between requirements and capabilities, providing a very attractive alternative to the used techniques up until now.

State of the art packages in industry

Packaging Trends

TO-XXX packages are the standard for medium power discrete devices. Over the last ten to fifteen years there has been a steady shift of growth from the TO-220 to the smaller TO-252 (DPAK) (see Figure 2). Large Aluminum (Al) wire bonding (5 to 20mil) is the main interconnect technique used in these packages. Ongoing developments in high performance wire bonding equipment and expertise, enables such devices to be interconnected reliably, with very high yield and at low cost. The main interconnect method for smaller, lower power packages, especially the SO-8, is Gold (Au) ball bonding with wire diameters in the range of 1 to 3mil. Advances in power chip technology enabled a trend towards smaller packages, with the same or similar current capabilities which the larger TO-XXX packages had only a few years ago. This makes sense from an economic viewpoint. For constant cost per wafer the price per die decreases with die size. A smaller and/or more efficient die allows the use of a smaller package, which means less material per device and therefore lower cost. Smaller devices require less area on a printed circuit (pc) board, contributing to cost savings on the system level. If all this enables new applications, economy of scale will bring additional cost benefits. Portable applications drove requirements that were difficult to achieve by the existing packages. They require very efficient power devices of very small size. In addition, high end applications in telecom and computing require the best possible electrical and thermal performance together with a small enough footprint. These needs spurred significant efforts to develop new packaging and interconnect designs, including wire bond free designs. While wire bond free designs address the performance requirements, most of them are proprietary, non-standard, with manufacturing costs remaining questionable. But, while performance is an initial enabler, cost must become inline with market needs. Hence the drive to standard, non-proprietary packages and interconnect technologies that provide good electrical performance, the needed footprint and reliability at a reasonable cost. Leadless SO-8 or PQFN-type packages fullfil this requirement.

Shift to smaller packages

Available Interconnects

Fine gold (Au) wire bonding becomes less and less effective even in small power packages where large wire diameters are desired. The high material cost runs against the need to increase the interconnect cross-section to accommodate higher current requirements. Its less expensive sibling, copper (Cu) wire bonding, reduces wire material cost and can improve electrical performance. However, the more difficult process caused by Copper’s material properties (hardness, oxidation behavior) adds additional (process, yield) cost. An area bonded Cu strap (clip) enables the desired electrical performance, but cost, flexibility and reliability remain questionable. The use of large aluminum (Al) wire in small power packages, appreciated in larger power packages due to its flexibility, reliability and low cost, only applies to some select applications. Its use is limited to wire sizes on the lower range of its capabilities due to package size contraints. In order for the broad range of mainstream applications to benefit from the recent developments in power chip technology, there is therefore a need for an overall effective interconnect technology with performance sufficient and cost low enough for the majority of applications.

PowerRibbonTM Bonding

PowerRibbonTM, developed by Orthodyne Electronics, represents the ultrasonic bonding of large Al ribbon in the range 20x4mil up to 80x10mil and is an evolutionary extension of large Al wire bonding. The round cross-section of the wire is replaced by the rectangular cross-section of the ribbon. Al material composition and mechanical properties are equivalent to large Al wire. The change in the geometry of the cross-section diminishes the horizontal flexibility, but increases the vertical flexibility of the interconnect. Horizontal flexibility, the capability to bond wires under large forced angles, is important to interconnect configurations with a complex structure such as TO-XXX packages and multi-chip applications. Vertical flexibility, the decoupling of thickness and width, i.e., the possibility to choose ribbon thickness and width independently, enables fitting a large interconnect cross-section, with a minimum number of ribbons, into the space given by an application. The user is now able to benefit from all the desired features offered by large Al wire bonding: its cost benefits, electrical performance, reliability and flexibility in small SO-8 and PQFN devices. Figure 3 shows that the layout of small power packages such as the standard SO8 or PQFN-type packages is perfectly suited for PowerRibbonTM bonding. The wide source lead along a large portion of the heat sink and die allows the ribbon to cover a large portion of the die with a minimal number of straight ribbons.

Layout of small power packages

Performance, Reliability and Cost

Performance

According to Table 1 the electrical resistance for 1mm length of 40x4mil Al ribbon is approximately 0.26mOhm, while it is approximately 11.35Ohm or nearly 43 times higher for 1mm length of 2mil Au wire or approximately 8.39Ohm or nearly 32 times higher for 1mm length of 2mil Cu wire. But the maximum number of fine wires that can be bonded in a standard SO-8 package is limited by the size of the source lead, depending on the width of the lead to 2o to 22 wires for 2mil diameter. For a die with approximate dimensions 140x100mils and a typical source metallization thickness of 4µm, the interconnect resistance (main loop plus spreading) of a configuration with two parallel 40x4mil ribbons with 2 stitches on the die each is approximately 0.5mOhm, which would be the equivalent to 18 Cu wires with 3mil diameter, the maximum number of wires that fits into this package. Considering that each bond joint represents a potential yield loss, this is an attractive alternative. If a device is operated at higher frequencies, inductance and skin effect in the interconnect need to be considered. Inductance primarily affects the switching behavior of a device. Skin effect reduces the effective interconnect cross-section, causing a significant resistance increase, and therefore power loss, at higher frequencies. Inductance is mainly a function of the geometry of the interconnect, with the length being the main factor. Ribbon offers similar inductance behavior as a strap and performance benefits compared to any round wire alternative.

Wire to ribbon conversion table for equal electrical resistance at identical loop length

Reliability

Although configurations with large bonded joints on the back and top side of the die can be made reliable by appropriate design and material choice, configurations with wire or ribbon interconnects on the top side of the die are inherently more forgiving and therefore more reliable, under operating conditions.

In several evaluation activities, standard and leadless SO-8 devices passed typical reliability tests, specifically (a) temperature cycling (500 cylces @ -65°C/+150°C), (b) high temperature storage (1,000 hours @ 175°C), (c) pressure cooker test (168 hours @ Ta=121°C, RH=100%, 15PSIG), and (d) Moisture Sensitivity Level 2 (MSL2) with standard preconditioning, all without any ribbon bonding related failure.

With their monometallic bond on the die, and the gate bonded with Al wire, Al ribbon bonded parts enable reliable operation up to chip junction temperatures of 175°C, which is a requirement in automotive applications.

Orthodyne 7200plus

Cost

All the different interconnect technologies have some weakness in at least one cost category. The high wire material cost is the main weakness of Au ball bonding. At $500 per ounce the Au material cost per SO-8 device with approximately 22mm total wire length is 1.7cent for 2mil Au wire, and 3.0cent for 2.75mil Au wire. Cost of 2x40x4mil Al ribbon in corrosion resistant quality and approximately 2x3.5mm length is approximately 0.2cent at low order volumes. An area contact design like Cu strap demands a non-standard metallization that requires additional process steps on the wafer level, adding additional cost on the order of $20 per wafer. For a 6” wafer with dice of 6mm2 size this adds approximately 1cent to the cost of a device. A strap specific to one die is inflexible and therefore costly. One strap type for several similar die sizes, limits the electrical performance. This makes such designs attractive for high performance applications, but less competitive for mainstream applications. Cost of Cu wire is approximately 10 times lower than for Au wire with the same dimensions. However, the Cu ball bonding process operates at a lower yield and lower process speed compared to the Au ball bonding process. Its material properties require special material handling and packaging including a cover gas to prevent the Cu from oxidation during and after the flame-off process. Despite these measures, the process seems to remain rather sensitive, especially for cratering when bonding over active area. In contrast, bonding a soft Al ribbon is a very gentle process, due to the geometry, and potentially even more gentle than Al wire bonding, which is known to be best suited for bonding over active area. In addition, Al ribbon configurations only require a few bonds on the die, offering much better conditions for minimal bond yield loss, compared to Cu wire bonding.

As an example, Table 2 gives an overview of ballpark figures for electrical performance, material cost and productivity of some select configurations for the standard SO-8 package.

Overview for performance characteristics of two sample die configurations for the standard SO-8

Conclusions

Large Al ribbon bonding is an evolutionary improvement of large Al wire bonding. It preserves most strengths of that technology and adds new ones, which make it very effective in interconnecting standard SO-8, and SO-8 size and smaller PQFN packages. Al ribbon bonding offers better electrical performance than fine Au at a much lower cost. It offers comparable electrical performance to Cu wire bonding at comparable cost. But for bonding over active areas its gentle bond process enables a higher yield than Cu wire bonding. Its electrical and thermal performance is comparable to the performance of Cu strap bonding but offers lower cost and higher flexibility. In addition, the monometallic Al-Al system on the die enables reliable operation up to 175°C chip junction temperature. All these strengths are possible with package layouts that follow established standards, and don’t require specific proprietary package and interconnect designs. Therefore it allows using well-known processes and equipment. In summary, Al ribbon bonding is the most attractive interconnect technique for small power packages for mainstream applications which require good electrical performance and reliability, at reasonable cost.

 

 

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