This article is aimed to describe the best methods for handling and mounting bare die pressure sensors. Merit Sensor manufactures all pressure chips on 4 inch wafers, which are sawn and delivered on Mylar film (foil tape). This foil tape is fixed to a metal wafer frame that is appropriate for the majority of automated die bonders (see Figure 1). If a unit is marked with a black ink dot, it is considered to be a bad unit.
Packaging & Storage
All wafers assembled on foil tape will be supplied in plastic clam shells (see Figure 2), which are subsequently inserted into an antistatic zip-lock bag. It is required that the bags be opened in clean rooms only and stored in a dark, nitrogen-filled cabinet as soon as it is opened. Wafers can be shipped either individually or multiple clamshells in a single plastic antistatic zip-lock bag. The label on each clamshell will include the quantity of good die, purchase order number (if applicable), the part number, and lot and wafer number (see Figure 3). The foil tape will also have the lot and wafer number written on it.
Storage Temperature is 19-26 °C: In a proper storage environment, the storage time of sawn wafers is about five years. Storage beyond this limit, or storage in a different or uncontrolled environment, may lead to picking problems at die bonding (sticking die) or unreliable wire bonds owing to corrosion of the aluminum bonding pads.
Handling of Wafers
All sensor chips are 100% electrically tested to guarantee that they conform to the datasheet limits. The wafers are visually examined to ensure that all sensors are completely defect-free. The pressure chips are RoHS compliant, and in the majority of cases, consist of a silicon/ glass stack that is electro statically bonded together.
- All wafers are mounted, tested, diced, and delivered on a metal wafer frame. Depending on the product, each wafer yields about 600 to 1600 pieces.
- Special care should be taken when handling the wafer as its surface is highly sensitive.
- Although cleaning is not required, the wafer should be opened in a clean room.
- It is not recommended to pick the die off the wafer frame with tweezers. The pressure chips should be picked up with a tool made of soft rubber with a vacuum hole in the middle that is bigger than the sensor membrane.
- The bonding force should not exceed 100 grams in order to prevent mechanical stress, which can lead to an unstable, drifting offset.
- It is essential to clean all the tools thoroughly to prevent any residue on the bonding pads, which could lead to reliability problems.
- For gage pressure sensors (hole on backside), ejector pins with 3 or 4 needles may be used to remove the die from the wafer tape.
- For absolute pressure sensors (no hole on backside) a single ejector needle will be adequate.
- Process temperatures greater than 225 °C should be avoided. If the maximum temperature is lower, the sensor will be more stable in a long-term.
Mounting of Pressure Chips
- Die bonding with hard silicone or epoxy will generally result in an unstable offset value and high TCO (temperature coefficient offset).
- The pressure chips are sensitive to mechanical stress, particularly sensors with full scale pressures below 1 bar. These pressure chips should be mounted using a soft silicone adhesive with a low hardness (A25 or lower) and a bond-line thickness of 50-100 µm. Particularly, care should be taken to prevent the adhesive from climbing up the inside or outside walls of the sensor die as this could lead to unstable output.
- All pressure chips have been optimized for long-term stability and the highest output signal. In order to achieve the best performance (temperature behavior, long-term drift, hysteresis), special care must be taken when mounting the die.
Attaching the Pressure Chips
- The bond pads on each pressure chip are at least 100x100 µm. The pad material is made of aluminum and has a thickness of 1-2 µm.
- An aluminum or gold wire can be used for wire bonding. A good thermo-sonic gold-ball bond, with 30 µm gold wire, will result in a ball shear force of >30 grams and a Pull Force of >6 grams.
- A soft ion free silicone gel with a viscosity of <1000 cps and no hardness should be used to protect the wire bonds. The gel can have a considerable impact on the sensor performance; hence, special care should be taken when making a selection. Merit Sensor has tested and currently uses Dow Corning Sylgard 527.
- The gel can be applied as drop on the sensor’s surface to simply protect the bond pads from corrosion. If additional humidity protection is necessary, then the entire area around the sensor including bonding wires can be covered.
- For gage pressure sensor, where the pressure is applied from the backside, Merit Sensor still recommends to protect the topside of the sensor with a gel to avoid corrosion of the aluminum bonding pad.
This information has been sourced, reviewed and adapted from materials provided by Merit Sensor.
For more information on this source, please visit Merit Sensor.