Time is right for bare die

Packaging technology as used with semiconductors is a means by which product can be easily handled and assembled into electronic systems. The technology has made significant advances over the years, however the fact remains that the actual silicon Die in the package is much smaller, lighter and has less mass than a packaged part itself. It was these need drivers of reduced weight, size and smaller mass that first inspired the use of product in Die form. As silicon technology has developed, packaging technology has not kept pace, which has opened up new opportunities for the use of product in Bare Die form.

In the early years Bare Die usage was driven mainly by Military Hi-Reliability type applications where weight, size and high reliability were driving the need for higher integration with the final system encapsulated into modules for easier qualification.

Later, with the growing use of electronics in Automobiles where hostile under the hood environments created real challenges for the engineers, solutions were required that were low cost while still achieving Military type reliability and qualification. This prevented the use of traditional ceramic packaging, so integrating product in Die form into a module was an obvious choice.

Today the need drivers in consumer and white goods applications have taken the use of Bare die into brand-new markets and it is these markets that are starting to drive the technology. How so? Take the washing machine, tumble dryer or air conditioning units as an example. In these systems the electronics components operate in very hostile environments. Heat, humidity and vibration are all present potentially reducing the life of parts housed in plastic packaging. Coupled with the fact that warranty periods for consumer goods are now extending, new solutions for packaging technology are becoming very popular and have opened up more possibilities for products to be used in Die form housed in customised packaging.

Product qualification is another important area. As the demands for higher qualification levels in Automotive continue, traditional packaging technology becomes a major issue. In such a cost sensitive market, ceramic packaging is clearly not an option, so what are the alternatives? In any system the most unreliable part is the connections. Mounting Die directly onto a substrate immediately cuts down the connections (packaged parts have connections from die - >lead-frame -> PCB) and Flip Chip/Flipped( takes this one step further effectively connecting the bonding area to the substrate/PCB.

Another challenge is that discrete packaging can undermine the potential performance of Bare Die products. Going forward the biggest limitation to technical performance is the packaging technology and not the silicon. Often the raw performance of the silicon is held back by the packaging technology. Take a MOSFET product targeted with a low RDS(on) (on resistance) as an example. As much as 40% of this RDS(on) resistance can be put down to the packaging (excluding the bond wires that are needed with a Die solution if not using Flip Chip technology). This is why companies are so focused on Bare Die, FlipFET, DirectFET and Modules. All of these technologies go some way in releasing the full potential of the silicon.

With more and more mobile applications, system integration is becoming key. ASSP (Application Specific Standard Products) and ASIC (Application Specific Integrated Circuits) have traditionally been a means by which more and more of a system could be integrated, reducing weight and space. However, these solutions can only take the issue so far. Going to Bare Die can take system integration even further. The reality is that if it weren't for these ways of improving system integration, cell phone technology would be far behind where it is today.

So the use of Bare Die can offer many benefits; but there are still a number of challenges in using the technology both from a supplier and user point of view. Handling Bare Die requires specialist knowledge, equipment and procedures and many companies are not sure how or where to start. Additionally, component suppliers have concerns over supplying what are effectively partly finished parts. Let's address the issues in turn.

Customer Adaptation
When International Rectifier (IR) first ventured into the Die supply market, the first customer to embrace the technology was a large OEM. As this was their first venture into this arena they decided to buy KGD (Known Good Die), as this was the highest level of performance testing available that assured the customer that the Die would perform to package part specifications and would eliminate one variable if there were problems in the production line. The first six months of production were a module yield nightmare for the company and engineers from IR were called to explain why all these KGD parts were failing. What was discovered through an audit of the customer's Die handling and processing techniques were that the customer was assembling these modules in a PCB production line area. They were treating the Die as if it were a standard package item. Once identified, the module assembly was moved to a more suitable environment and the yields increased dramatically. The customer is still an important Die customer for IR.

Selling Die is part of a two-way relationship. It is important that opportunities are carefully screened to ensure the customers have the capability and knowledge to handle Die. Even where there is no existing in-house capability, there are ways and means to get companies properly qualified to handle and process Die and for those that don't want an in-house capability, the solution may be found in using external module makers who can offer a complete module solution, gently easing the customers into the Die use arena.

Supplier Adaptation
There can be some reluctance of suppliers to make their products available in Die form. There are a multitude of reasons for this including:
1 Confidentiality - supplying Die product causes uneasiness because construction of the Die is exposed to scrutiny. However, this knowledge can also be obtained with assembled devices. Providing access to die sizes and yield information (if the product is supplied in whole wafer form or die on film, customers can see the yield that is being achieved) is also a concern as it can indicate the wafer costs associated with a Die part
2 Specifications - concerns about what operating specifications can be committed to. When Die is probe tested as a whole wafer, it cannot be tested at the same level as a discrete packaged part.
3 Warranties - the careful handling required for Die by customers can open itself up to many claims of faulty product when in fact it could be customer mishandling or assembly issues, which are difficult to prove.

These are all valid concerns that need to be addressed internally within suppliers before they can commit to supply product in Die form.

Other considerations
One important issue here for the customers is how serious the supplier is committed to Die sales. Sometimes Die sales are seen as fabrication facility fillers - a great way to keep a facility loaded when capacity is available but dropped as soon as capacity becomes tight again. Often companies that support this approach quote very aggressively when they want this business creating artificial aspirations on the price that cannot be supported when the market changes.

Looking at the benefits of Bare Die it is clear that the motivations for using this technology are no different than that of going to ASIC's. Size, weight, reliability, performance and costs are common drivers for both types of technology making the use of product in Die form a viable alternative especially when development costs, development timescales and risks are important considerations. With a plethora of module companies, OEM's are being increasingly exposed to Die technology in one form or another and gradually they are beginning to embrace the technology directly themselves. Like many technologies, Military requirements initially drove the demand. However, with the growing need for improved performance, reliability and miniaturization, the possible applications for the use of Die are virtually endless and the use of Die has now really come of age.