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Getting Help with Bare Die
Probing individual die is not always cost-effective. (Photo: PASS Ltd. Hong Kong)
By Curt Olsen, Director of Sales, ES Components, Sterling MA
Specifying and obtaining the correct bare die to build hybrid microcircuits and multi-chip modules presents some specific problems. Dominant design factors here are hermeticity, 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 discretes are assembled in packages that use a braised clip for the top contact attach. 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. 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. For devices with high minimums, the die distributor can possibly offer alternatives that provide the equivalent function from their stock.
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 microcircuits 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.
Finding the Product
The buyer or component engineer involved in the specification or procurement of bare die will face a number of unique questions at the time of initial inquiry when looking for product support:
Do you want bare die, or are you looking for a packaged part? Often the bare die part number is significantly different from the packaged device part number recognized by the market. The potential user has no way of knowing this from the packaged part data sheet. We ask this question whenever the customer specifies an obvious package part device, unless the user specifically indicates "die" in the inquiry.
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
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? Package 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 toward sawn wafer on film to reduce cost and enhance manufacturing throughput. However, sawn wafer on film has shelf life limitations which demands 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.
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.
Another problem area where the die distributor adds value is the recurring issue of geometry changes. Semiconductor vendors repeatedly implement "die shrinks" which provide more die per wafer, hence more product, and more margin. Occasionally these shrinks cause pads to be relocated, which can create a wire-bond cross-over or extend a bond wire longer than recommended for the wire type. The vast majority of these geometry changes do
fall into the category of a change in form, fit, or function relative to the package part. Therefore, no PCN is provided to the user community. You don't know the geometry has changed until the shipment is received and the die is different.
Hybrid circuits use several bare die. (Photo: University of Bolton, UK)
At ES Components, we have a complete library of virtually every die geometry in our inventory or that we have ever processed. Whenever a new lot of material is received, the first thing we do at incoming inspection is look for any changes to the geometry on file. If we are supporting an ongoing production requirement, we can quickly provide notification of a change to the user. If the new geometry creates a problem for the user's assembly, we can possibly get the die manufacturer to support an end-of-life (EOL) procurement of the old geometry, or find an alternate vendor that more closely meets the geometry required. The user does not get the unwelcome surprise at incoming, or worse, on the production line.
Once a BOM is defined and production starts, ES Components offers a systems driven Vendor Managed Inventory (VMI) capability that minimizes the unique pipeline issues associated with the supply of bare die. This includes the acquisition of wafers or die, probe and LAT as required, storage of the qualified material in the appropriate environment, processing on demand as directed by forecast, particularly where sawn wafers on film is required, and virtually 100 percent on-time delivery to one or more manufacturing locations simultaneously.
The system can be easily modified to reflect unique user requirements. Advantages include greatly reduced procurement logistics on the part of the user, a significant increase in inventory turns in manufacturing, and if forecast properly, no work-arounds due to material shortages. A VMI pipeline also provides the user with advanced notification of pending changes to the product such as geometry changes or end-of-life notification, and changes in lead time are typically seamless. A Vendor Managed Inventory is particularly beneficial for long-term programs where the design is fixed and the ability to forecast is reasonably accurate, and can even be applied to a small subgroup of parts that demand special attention.
Die Distribution came into being in the early 1970s. It continues to flourish in the supply of these "specialty products" that extend the list of user considerations beyond that normally encountered in the procurement of packaged devices. These companies were truly pioneers in the evolution of Added Value Distribution and provide a tremendous service to a broad array of industries and markets, including Aerospace, Military, Avionics, Medical Implant, Automotive, and high-end Industrial where bare die is the optimum choice for a given design.
Contact: ES Components, 108 Pratts Junction Road, Sterling, MA 01564
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