Die skewing: due to the continued growth of flip-chip technology in next-generation products, avoiding die skew is essential.

Problem

Flip chip devices are incorporated into new products, within many sectors of the electronics industry, with increasing regularity. A number of design-, material-, process- and equipment-dependent variables must be understood to ensure the technology is implemented and sustained successfully. For example, the handling and placement of bare die creates new challenges not typically experienced with standard surfacemount assembly. A customer recently asked, "We're seeing an increase in the number of skewed die during our flip chip assembly process--what could be causing this problem and how do we correct it?"

Problem Solved

Die skewing is a process problem that can occasionally occur when handling bare die. Several factors in the flip chip assembly process can cause this placement defect, including improper equipment setup, incorrect processing parameters and incompatible assembly materials. In addition, placement accuracy may be affected not only by the die placement process, but also by other manufacturing steps, such as flux application, board transport and solder reflow. Quickly determining and eliminating the variable contributing to the specific skewing problem can, at times, be challenging to the manufacturing engineer.

For the specific case of die skewing mentioned previously, a series of methodical steps were followed to determine the cause of the problem and to provide a solution. Working with the customer, initial emphasis was placed determining which process step was causing the problem. Process checks were made on the upstream and downstream processes, primarily flux application and solder reflow, to determine the potential impact of each process. After concluding that processes and equipment configurations were within specification limits, and ensuring that all independent and dependent variables were correct, an observation of die placement was made at the customer facility. The observation revealed that different nozzle types resulted in significant skewing effects across various die lots.

The placement process, with respective variables and conditions used at the customer facility, was then simulated on a surface-mount assembly line. Using the customer's actual product board and various die lots, high-speed video was taken during the placement process when using both nozzle types. The video revealed instantaneous sticking of the die to nozzle type B (rubber) at the moment the die was released, but no die sticking when nozzle type A (plastic) was used. Placement was performed with no flux present to accentuate any cases of die-to-nozzle sticking. A rigid board support was used to minimize the effects of spring boarding, information was also gathered to determine the interaction of each nozzle type to various die lots.

Contamination on the die and nozzles, and the surface finishes of the die and nozzles, was thoroughly investigated. Characterization analysis, including solvent extraction, scanning electron microscopy (SEM) and Fourier transform infrared spectroscopy (FTIR), was performed on samples of both nozzle types (new and used) and the various die lots. Analysis showed that backside cavities were present on the failing die lots (approximately 30 mm in diameter and 8 mm deep). Organic compounds were also found--primarily bonded and free oxygen-hydrogen (OH) functional groups. Such conditions could lead to die sticking issues, either by excess adhesion or by the localized effects of vacuum. Additionally, FTIR analysis revealed functional groups, indicative of free OH functional groups, that may be found in organic acids, alcohols and solvents, on used nozzles (both A and B). The likely cause of die sticking can be attributed to an interaction, or reaction, of the functional OH groups, causing excess adhesion between the die and nozzle. New nozzles were installed on the placement machine, and the skewing problem was immediately corrected.

In this particular case, the resolution of the die skewing problem was a simple correction. The condition of the pick-up nozzle surface is critical to obtaining the proper adhesion and separation between nozzle and die. Proper nozzle surface cleaning methods are crucial to ensuring nozzle (and placement) performance.

S. Craig Beddingfield is senior market manager and Brian Lewis is advanced process development engineer for the Advanced Assembly Technology Center at SiemensDematic Electronics Assembly Systems. Inc., Norcross, GA: e-mail: [e-mail]; [e-mail].