Automated Optical Inspection

Flux Inspection with UV Fluorescence AOI: Making the invisible visible

As Seen in Advanced Packaging, September 2004 by George T. Ayoub

Flux inspection has been a challenge for Flip Chip and BGA assemblers due to the inability of inspection systems, including AOI (Automated Optical Inspection), to accurately see the material and therefore be able to inspect it while maintaining line speed. Specifically with respect to flip chips, the inspection of flux is an important part of controlling the process and can prevent costly mistakes from happening. Fortunately, a machine vision solution utilizing UV (Ultra Violet) illumination, which has been developed over many years of research, is able to detect defects from flux deposits. This technique, which has now proven successful for more than three years on high volume manufacturing lines, replaces the visible AOI light with specialized UV light that matches the properties of the substrate and flux, in order to achieve optimized inspection results.

♦Importance of Inspecting Flux in the BGA/CSP Assembly

Flux plays a critical part in the process dynamics of BGA/CSP package assembly. A vast range of defects in the final assembly can be traced back to poor flux or paste deposition. For example, some of the defects in the final assembly derive from poor flux alignment with respect to the intended pads, insufficient thickness/amount of the flux material, excessive amount of flux, or from smearing. The detection of these pass/fail types of defects (attribute data) at an early stage of the process reduces the assembly cost significantly. Moreover, many manufacturers would agree that it is important to control the process of flux deposition by means of relevant measured variables in order to detect trends and prevent defects from happening in the first place. This requires a system that is able to measure the key variables of the process (variable data). By providing real-time information on key process parameters, manufacturers can take corrective action and prevent scrap and production loss.

♦Technology Challenges: Making the invisible visible.

The process of flux inspection flux has been a challenge to AOI manufacturers due to the inability of visible light to image well the flux material and therefore to be able to inspect it. Both high and low angle illuminations in the visible suffer from poor signal to noise ratio of the flux with respect to the background. However when the flux is illuminated with UV lighting, it fluoresces in the visible and the signal can be captured by means of proper filters designed to eliminate any background light not emanating from the fluorescent flux. Under these conditions the signal to noise ratio between the flux and the background is enhanced significantly. The key to obtaining a good signal to noise ratio is the proper design of filters and illumination -which are proprietary – that are adaptable to the flux itself and to the background material (ceramic and possibly FR4) and at the same time able to eliminate the visible background light. (See Exhibit “A” – Images under visible light versus UV light.)

♦Challenge: Speed and resolution for an in-line system.

Image acquisition parameters play an important part in the capability of the system. Two important parameters are the speed of inspection and the proper optical magnification (resolution). Both are related because speed of acquisition is inversely proportional to the number of pixels acquired, which in turn varies linearly with the square of the magnification. Moreover, illuminating small areas require a large amount of light and an adequate number of pixels on target. The requirements to keep up with very fast cycle times coupled with the high resolution were met by means of utilizing multiple cameras heads, proper illumination utilizing UV diodes and specialized electronics. Multiple camera heads (in this case three are used) extend the Field of View from square to rectangular shape, while at the same time not sacrificing the resolution. The specialized electronics allows the camera to acquire in parallel and matches their speed to the processor computing speed. Utilizing UV diodes assures longevity and stability of the system over time, which is extremely important when an inspection program need to run without modification on different systems or production lines.

The system is also capable of measuring the paste parameters under UV and/or regular visible light making it extremely useful for controlling the paste deposition process at the same time. The algorithms, that extracts position, area, pad area coverage and brightness are based on blob analysis and are only applied in the areas of interest.

♦Defect detection, measurements variables and SPC

Utilizing UV fluorescence techniques, the system in operation goes beyond the detection of the pass/fail defects attributes, and assists in enhancing yield by means of SPC techniques on measured variables. It measures the position, area, pad area coverage, and brightness of the deposited flux. Brightness is measured by calculating the median gray scale of the paste blob. Although it is ideal to utilize the height and volume of the flux, these measurements with the UV technique utilized here, are seen to be dependent on the properties of the flux and the background where the flux is being deposited, and cannot be trusted in all cases as absolute measurements. There are good logical reasons backed by experiments confirming that brightness correlates with the height of the flux. In effect, the brightness depends on the amount of fluorescent material in the flux and therefore should vary linearly with the volume. However this linearity is not always certain, but depends on the environment. Therefore, care should be taken when interpreting the measured brightness since other materials may fluoresce also and add to the noise. The method described has proven to be effective in the production environment, utilizing brightness along with position, area, and pad area coverage as measurement parameters, thus providing a logical and adequate means for controlling the final quality of the process by means of SPC methods.

In the production environment, real time process control has proven to add value to the process by following trends and preventing defects from happening, (See Exhibit “B” – Real-time data charts) and has been an integral and critical part of the system. Depending on the alarm setting, the system is able to stop the line or turn on a yellow or red light for visual feedback to the operator.

♦Conclusion

The described technique has been proven for over three years of inline inspection. The system is process capable with GRR in the range of 2.5% to 8. It is able to keep up with a relatively fast production line speed while achieving a false call rate in the range of 10 to 20 ppm and a false accept rate less than few ppm. By containing defects at this early stage and by controlling the trends with SPC, good results have been achieved. Future work planned is to keep enhancing the signal to noise ratio and to extend the application of this technique to different substrates and flux types.

References
Reliability and Yield in Flip-Chip Packaging, Alan Lewis, Ed Caracappa, Lawrence Kessler, 1998_11_hdi_flip_chip_reliability.pdf