Optimizing Bare Die Selection for Reliable Hybrid Microcircuits

Unless you have been doing this for a good number of years you will quickly find out that there is a lot more involved than just searching for a Part# on the Internet. It’s especially frustrating to find out that in a world of “packaged” semiconductors that Bare Die commerce is perceived as a bother. The trick is understanding the differences between packaged parts and bare die. 

What is Bare Die and how do I buy Bare Die? 

Manufacturers produce a wafer that yields the die. After testing the wafer, individual die are separated from the wafer and assigned a part number and then shipped to a bare die distributor. Here, samples from a die lot are packaged to expedite Lot Acceptance Testing (LAT). Additional testing is usually done through a method of Known Good Die (KGD).

First, find a trusted Bare Die supplier that can provide you with the components in die form. Their expertise and advice will help you in the buying process. You may need Certificates of Conformance, Lot Traceability, Source Control Drawings, Geometries, Specific Testing performed. It can be very confusing.

Manufacturers produce a wafer that yields the die. After testing the wafer, individual die are separated from the wafer and assigned a part number and then shipped to a bare die distributor. Here, samples from a die lot are packaged to expedite Lot Acceptance Testing (LAT). Additional testing is usually done through a method of Known Good Die (KGD).

At the distributor’s facility, the die is visually reinspected to make sure that the die that you purchase looks flawless and will function well. In some instances, additional electrical testing is also performed for screening of specific electrical parameters.  Current die geometries must be obtained and any changes that have occurred must be forwarded to the end customer for review and approval. Often times samples are required by the end customer. Depending on the device, the distributors can usually provide the required sample. Sometimes though, the part is not available from the manufacturer in die form, or if the manufacturer agrees to provide the part, the minimum purchase quantity may be much, much higher than the end customer requires. Always keep in mind that everything is NOT available in die form.

Specifying and obtaining the correct bare die to build hybrid microcircuits and multi-chip modules presents some specific problems. Dominant design factors here are hermetic sealing requirements, size, and weight. However, the special needs related to the supply of bare die continues to mystify much of the electronics community beyond the handful of hybrid manufacturers specifically focused on this market.

Most hybrids are designed around the characterization of packaged parts. Once samples are received, the function designer builds the breadboard, runs characterization data, and then asks the hybrid designers to create a hybrid or multi-chip module to simulate the function. Here is where the die distributor immediately begins to add value.

Taking the Bill Of Materials (BOM) from the customer, we immediately determine which of the requested parts are not readily available in die form or require added value processing. First, some semiconductor devices are not available as bare die because the manufacturer simply elects not to offer the device in die form. Many newer power discrete devices are assembled in packages that use a braised clip for the top contact attachment point. The top metal used to facilitate these packages is not suitable for wire bonding or soldering, so the manufacturer will not offer this die to the market.

In this era of large wafer diameters of 6-in. or 8-in. and larger, the manufacturer may elect not to 100 percent-probe smaller die at the wafer level. The manufacturer does sample probes only, and accepts the predicted yield loss during 100 percent test of the packaged part.

In some cases, parts that have no prior history in the die market may have prohibitively high minimum order requirements in die or wafer form from the manufacturer, making it impossible to sample or support prototype builds.

The die distributor can address each of these problems and either perform the required added value or recommend a list of alternatives that are more readily available. In the case where a manufacturer will simply not quote a bare die, the die distributor often has access to alternative sources that can offer die with similar functions.

Suitability for Wire Bonding

Where the top metal is not suitable for wire bonding, it is usually not difficult to find a similar device within the vendor portfolio that is offered in die form with the preferred metallization. In the case where 100 percent probe is not performed by the manufacturer, the die distributor can offer this service using in-house capabilities or outsourcing to an approved test lab. This also involves the distributor performing a Lot Acceptance Test (LAT) after probe, to provide objective evidence that the probe successfully met the specified requirements.

Stocking and Inventory Control

At ES Components, we have proactively identified a long list of popular devices throughout our line card and established an “off-the-shelf” inventory. This inventory is automatically replenished based on a predetermined min/max rather than market demand. An off-the-shelf part is immediately available for sampling and has virtually no Minimum Order Quantity (MOQ) when supporting prototype procurements. The products offered include a variety of analog micro-circuits and a compete portfolio of popular discrete die and thin film resistor chips. We use this preferred inventory to quickly offer the hybrid designer alternatives that are readily available and provide a cost-effective solution. If the customer is unable to use one of these devices, we then support the procurement of a different required device.

Are you soldering to the top contact or wire bonding?

This is a must if there are any power discrete devices on the BOM.  Many discrete die are available with either option. The rest are only offered with one or the other of these two options. If only solderable top metal is available, and there are no alternative devices, the die distributor can possibly offer a sub-assembly using a Copper or Molybdenum disk with suitable top plating to meet wire bond requirements.

How are you planning to attach the die?

The vast majority of power discretes are available from the manufacturer with solderable back metal only, some with either solderable or gold back metal. If the user is planning to use epoxy attach, it is highly recommended that they not use die with solderable metals. Gold back metal is recommended for either eutectic or epoxy attach. At ES Components we have developed and qualified the ability to remove solderable back metal from wafers and deposit gold for devices where this option is not available from the vendor. Silicon back die are generally epoxy-attached and if a eutectic attach is required, a gold alloy preform is needed.

How will you want this bare die packaged for delivery in full production?

Packaged parts are typically supplied in production quantities on tape-and-reel with nearly infinite shelf life. Waffle packs are the most common medium for shipping bare die, but high volume automatic assemblies are moving towards sawn wafer on film to reduce cost and enhance manufacturing throughput. However, sawn wafer on film has shelf life limitations which demand special management of the supply pipeline. A die distributor can manage the pipeline so that probed unsawn wafers are kept in stock in the appropriate environment and sawn wafers on film are limited to only those wafers needed for short-term production. If there is a delay in manufacturing, the unsawn wafer inventory remains pristine and the shelf life issue is confined to a small subset of the inventory.

Summary

By answering these questions at the time of initial contact and early in the design process, it’s possible to save the user time and money by avoiding the need to modify or redesign the function based on sample evaluation possibly weeks or even months later. We can also properly code the die part number to capture all the pertinent information for future reference.

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