What are the causes of chip deflection?

1. Problem
Incorporating flip chips into new products is an increasing trend in many areas of the electronics industry. Therefore, a multitude of design, material, process and equipment related variables must be understood to ensure the successful implementation and survival of this technology. For example, the handling and placement of bare chips creates new challenges not usually encountered in standard surface mount assembly. A customer recently asked, "We are seeing an increase in the number of skewed chips in our flip chip assembly process - what might be causing this problem and how can we correct it?"
2. Problem Solution
Chip skew is a process problem that can occasionally occur when handling bare chips. Several factors in the flip chip assembly process can cause this placement defect, including improper equipment settings, incorrect process parameters and incompatible assembly materials. In addition, placement accuracy is affected not only by the chip placement process, but also by other manufacturing steps, such as fluxing, board handling and solder reflow. It is often a challenge for manufacturing engineers to quickly determine and eliminate the variables that cause this particular skew problem.
For the special case of chip skew mentioned earlier, we follow a series of methodological steps to find the cause of the problem and provide a solution. Working with this customer, the initial focus was on determining which process step was causing the problem. Process checks were performed on both upstream and downstream processes, primarily fluxing and solder reflow, to determine the potential impact of each process. After concluding that the process and equipment conditions were within specification and ensuring that all independent and dependent variables were correct, we observed die placement at the customer's facility. Observations showed that different nozzle types had a significant skewing effect on various die lots.
The placement process was then simulated on a surface mount line with the same variables and conditions as those used at the customer's facility. Using the customer's actual products and various die lots, high-speed filming was performed during the placement process when both nozzle types were used. The film showed that the die adhered to the B-type nozzle (rubber) at the same time as the die was released, but there was no die adhesion when the A-type nozzle was used. The placement was performed without flux to highlight the die adhesion to the nozzle. The board was rigidly supported to reduce the board elastic effect. Information was also collected to determine the interactive effect of each nozzle type on the various die lots.
A thorough investigation was conducted on the contamination on the die and nozzles, as well as the surface finish of the die and nozzles. Samples of both nozzle types (old and new) and various die lots were characterized by solvent extraction, scanning electron microscopy (SEM), and Fourier transform infrared spectroscopy (FTIR). The analysis showed that backside pits were present on the failed die lots (approximately 30 mm in diameter and 8 mm deep). Organic compounds were also found - primarily adherent and free oxyhydroxide groups. This condition can lead to die sticking problems, either due to excessive adhesion or due to local effects of the vacuum. In addition, FTIR analysis revealed compound groups on the old nozzles (both Type A and Type B), indicating the presence of free oxyhydroxide groups, which may be found in organic acids, alcohols, and solvents. The possible cause of die sticking is due to the interaction or reaction of the oxyhydroxide groups, resulting in excessive adhesion between the die and nozzle. The new nozzles were installed on the placement machine, and the skew problem was immediately corrected.
In this particular case, the solution to the die skew problem was a simple correction. The condition of the nozzle surface is critical to obtaining proper adhesion and separation between the nozzle and the chip. Proper nozzle surface cleaning method is critical to ensuring the performance of the nozzle (and placement).