Quick – Tell Me How To Handle Bare Die

Handling Bare Die can be tricky.The following should help raise the sensitivity and awareness of special physical effects which could harm the quality and yield of the production of bare die.

Sensitivity

Integrated circuits are sensitive in respect to

• Electric Fields and Overvoltages

• Mechanical Damage and

• Surface Contamination

Electrical fields

Chips are protected against ElectroStatic Discharge (ESD) up to a certain level defined in the Quality and Reliability Specification (QRS) and circuit specification. Depending on the design of the device protection the ESD protection can reach values up to some kV.

Electric fields are mostly generated by moving objects or persons. A person, for example walking on a carpet, can easily be charged up to 35 kV (see Table 1). But not only high voltages even small electric charges can be sufficient to damage sensitive electronic components.. Devices which have been exposed to a certain electrical field must not be destroyed but the reliability of the IC may be affected. Therefore it is necessary to handle bare die in a electrostatic protected environment

Surface Contamination

Interconnection and molding steps are very sensitive to microscopic and macroscopic surface contaminations. Polluted surfaces can be the reason for issues during assembly or lead into long term instability.

Work Place Requirements

Clean room

Bare die must always be handled in a clean room environment of at least class 1000 (ISO 6) or better. A clean room is a environment that has a controlled level of contaminations, such as dust, aerosol particles or microbes. They are used in laboratory work and in the production of precision parts for electronic or aerospace equipment. The level of contamination is classified by the number of particles at a certain size per cubic feet or cubic meter.

The external and internal air for the clean room is filtered through high efficient filter systems to exclude particles and remove internally produced contaminations. Entering the clean room has to be done by passing an air or vacuum shower to remove adherent particles.

Staff, working in the clean room has to wear special protective cloths: heads, face masks, gloves, boots and cover-alls

All equipment used in a clean room has to be designed to avoid emissions. Special supplies are available for example:

• Clean room ball point pen, which have low-sodium ink to provide protection from ionic contamination (pencils and erasures are not allowed in clean rooms)

• Clean room paper, which is designed not to emit particles

• Clean room binders, which are solvent resistant

Electrical grounding

Every possible device which could be in contact with the die must be on the same electrical potential, this is also true for the operators dealing with the equipment. To achieve this, every body and everything must be grounded to same potential. Dedicated

equipment is readily available:

• Grounded workbench surface

• Conductive carpets

• Grounded chairs

• Low impedance place mats

• Grounding wrist straps

• ESD save shoes, coats, gloves and finger cots

The goal is, that nothing can get charged up due to motion or separation.

Special working behavior

• Access to clean room areas should be limited to only persons necessary for the area operation.

• Eating, drinking, chewing gum and smoking is not allowed in clean rooms.

• Nervous relief type mannerisms such as scratching head, rubbing hands or parts of the body, or similar type action are to be avoided.

• All material can only be moved from one clean room to another in the same or a lower clean room class, but never in the other direction.

Die Handling

Bare die must be handled always in a class 1000 (ISO 6) clean room environment: unpacking and inspection, die bonding, wire bonding, molding, sealing. Handling must be reduced to the absolute minimum, un-necessary inspections or repacking tasks have to be avoided. (Assembled devices do not need to be handled in a clean room environment due to the product is already packed well.) Use of complete packing units (tray, FFC, tape and reel) is recommended and remaining quantities have to be repacked immediately after any process (e.g. picking) step. To avoid contaminations and damages (scratches, cracks)

• Die or wafers must never be handled by bare fingers

• The active side of a die should never be touched

• The mechanical pressure has to be limited

• Do not store and transport die outside protective bags, tubes, boxes

• Work only in ESD save clean room environments

Special tweezers are suitable for grabbing die and wafers on its edge. Vacuum tweezers are used to move die from the packing to the target.

Transport & Store Conditions

Wafers and bare die are delivered in dry pack (see Figure 18). The dry condition of dry pack is guaranteed for one year. This does not affect the shelf life (see Section 6.3.3) if the transport and store conditions, described hereafter, are kept. If the conditions of Section 6.3.2 cannot be kept, the dry pack has to be renewed after 1 year.

Transport conditions

General transport conditions

During transport, the packing and the products have to be protected, among others, against extreme temperatures, humidity, direct sunlight, and mechanical forces. The temperature has to be between 8 °C to 60 °C. The total transport time should be as short as possible. When the transport time exceeds five days the transport conditions shall be the same as the store conditions described in Section 6.3, or the products has to be sealed with inclusion of a dry-agent.

Conditioned air transport

For dry pack, conditioned cargo rooms are mandatory for air transport. The temperature has to be between 8 °C to 45 °C with an average humidity of 16 %. The air pressure has to be between 700 hPa to 1060 hPa.

General store conditions

Secure and clean store areas shall be provided to isolate and protect the products. Conditions in the store areas shall be such that the quality of the products does not deteriorate due to, among others, harmful gasses or electrical fields.

The following conditions have to be kept:

• Temperature between 8 °C to 45 °C

• Humidity between 25 % to 75 %, no condensation under any condition is allowed

• No exposure to direct sunlight

Die are best stored in the package as delivered. Chips on film frame carrier (FFC) will stick stronger on the foil with time and will require more effort to pick them off the foil. In worst case some residual foil (glue) might stick on the rear side of the die.

Store conditions if not packed in dry pack

If wafers and bare die are not packed in dry pack, they have to be stored under following conditions: inert gas, dry air, dry nitrogen or in so called nitrogen flow boxes, relative humidity below 30 % and temperature between 18 °C to 24 °C.

Shelf life

The shelf life is the possible storage life before the product is used. Typically shelf lifes are:

Bare die in tray — 3 years

Wafer on FFC — 0.5 years

Unsawn wafer — 3 years

Now that we know how to handle bare die:

Watch How They Make Silicon Wafers and Computer Chips

Reference: NXP Semiconductors