Common Issues of Die AttachOctober 28, 2021
The physical damage, fractures and adhesive contamination anywhere on the die are considered common issues of the Die Attach process. The following problems/issues are commonly encountered with the Die Attach process:
1) Die Attach voids
2) Die Lifting/Tilt
3) Die Cracking
4) Solder Shorting
5) Die Scratching
6) Die Metallization Smearing
7) Lost Die
8) Die Placement
9) Die Rotation
Die Attach Voids:-
Voids are the small areas in the Die Attach area that are filled with a mixture of gases and other environmental variables, such as, air, moisture and dust. After the die placement, there could be a significant amount of waiting time before the subsequent curing process. It can result in a high level of moisture absorption in the Die Attach pastes.
While the organic substrates may absorb moisture, moisture may also be present on the metal leadframe surface. As the temperature increase during curing, absorbed moisture will evolve as steam to cause a void. The below are the X-ray images of Die Attach Voids:-
Voids in the Die Attach joints have a significant impact on cracks, electrical characteristics and thermal conductivity of the device. These voids increase the thermal resistance and decrease the electrical isolation capability which affects the quality and reliability of the device’s performance. The below are some techniques typically used to detect voids:
- Scanning Acoustic Microscopy
- Electron Microscopes
- Glass Die with an Optical Microscope
Die Lifting/Die Tilt:
The separation of the die from the lead frame after the Die Attach process is referred to Die Lifting/Die Tilt. This happens due to excessive Die Attach voids, incomplete Die Attach coverage, insufficient Die Attach curing, Die-back contamination, etc.
One possible source of lifting is the adhesion of the silicon chip to the Collet/Pick Up Tool. The adhesion may occur from high contact pressure between Collet/Pick Up Tool and the die or the irregular control of vacuum in the Collet/Pick Up Tool.
Another possible source is the elastic potential energy in the substrate or die backside metals. If the metal layers of the die backside are not flat, it may experience elastic deformation under the bond force. If the die is not well placed, the leadframe may deform elastically under the bonding force.
Die Crack occurs when the ejector pin contacts the backside of the silicon chip from the wafer tape during the die bonding process with a higher level of pressure than intended. The excessive mechanical stress can cause fractures in any region of the die. Most Die Cracks are caused by voids and incomplete fillet formation and imperfections in the Die Attach material. There are multiple factors to consider in minimizing the risk of die fractures/cracking in semiconductor packages i.e. silicon die strength, residual stresses after Wafer Dicing/Die Bond/Die Attach/Molding process, etc.
Sometimes die backside defects include micro-cracks and chip-outs caused by the wafer dicing process. This happens due to the effect of parametric settings of wafer dicing equipment or improper placement of diamond cutting blades by which the chips are diced. To detect the Die Cracks, the X-ray microscope uses electromagnetic radiations in the soft X-ray band through the specimen to produce a projected image.
Solder Shorting occurs when any solder overflow touches the leads and creates electrical shorting between them. This happens only during the Solder Dispensing Process. In Solder Dispensing Process, high volume of solder wire is dispensed through a Solder Dispensing Assembly of a Die Attach machine to form a specified solder pattern on the heated leadframe or substrate.
The Die is picked by a die bonder arm from the wafer holder and mounted on the molten solder before it cools down in a controlled ambient temperature environment. During the molten stage, an overflow of solder occurs as depicted in the image below.
The below image represents the high volume of solder wire is dispensed on the heated leadframe.
Die scratching mostly occurs when mishandling of diced wafers by an operator through tools like tweezers, needles or inducement of mechanical changes by the machine due to changes in parametric settings or mechanical failure of the part.
This happens due to inappropriate training of users, worn-out tools, contaminated tools or use of improper tools. The below images are showing the example of scratches on a full wafer and a single die.
Die Metallization Smearing:
Die Metallization is classified as a layer of metal or spot on the surface of the die. If an operator picks and places the die with contaminated tools, dirt/moisture would come into contact over the surface of the die.
After the Die Attach process, if no die is detected on the substrate, the die is said to be lost. This issues arises due to the Collet/Pick Up Tool having a vacuum issue or an ejector needle that is out of alignment during pickup. The below image shows the lead frame with Die Lost.
After the Die Attach process, if the die is not placed correctly on the substrate, it is referred to as a Die Placement error. These issues arise when mechanical failure causes changes in the motion of the bond arm in the X-axis or Y-axis, as well as changes in the indexing time of the leadframe in the indexer assembly. The below image represents the disturbance in the placement of the die.
Die Rotation occurs when the die is connected to the substrate at an incorrect angle after the Die Attach process. It occurs when the vacuum provided by the Collet/Pick Up Tool is insufficient, and the position of the bond arm deviates from the predetermined define angle.