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An Interesting Approach to Yield Improvement
The approach involves spending a little more money than normal at the start of project and the results show savings of many times more than this outlay.
Technical Paper
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Authored By:
Keith Bryant, Technology and Business Consultant
and Milan Popovics, Enics Elva.
Summary
Whilst many forward-thinking companies invest time, effort and cost into up front work to fix snags which would lead to issues with yield during production, this paper shows the efforts of a company who take things further. With increasing pressure on cost reduction within our industry, companies are looking ever more closely at their manufacturing process. In order to remain globally competitive and even to succeed in their local market every dollar saved here helps the bottom line. However, in many areas there is a danger that lower price equals lower quality and therefore actually higher costs in the end.
The approach here involves spending a little more money than normal at the start of project but less than hundreds of dollars and the results show savings of many times more than this outlay. However, it is acknowledged that this does take a little more time to get the job onto the shop floor. The key to this methodology is that it needs the time and effort of a skilled team and time on a production line before the job is started. But as the paper shows it really does improve yield, reduce cost, save the potential issues around repair and gives better reliability.
In essence the results of the printing process are analysed, after the components are placed, using x-ray and these results compared to the results after reflow soldering. The resultant pre reflow solder paste shapes are impossible to see with the naked eye or by lifting the components, as the paste would not release evenly. This allows the engineer to determine how differences in printed paste shape and volume react when components are placed on them and how ultimately this affects product quality.
Post reflow problems including mid chip solder balls were found to be common faults, as were issues under BGA’s including insufficient solder and shorts. The product is run on a “real line” and the results evaluated. Improvements are then made to the stencil design and other key process parameters to ensure that when in production the board is producing acceptable yields.
Conclusions
This is a technical paper so not really concerned about commercial aspects. But it must be said that in addition to the improvements in quality and reliability of the products produced due to the lack of rework and repair. Plus, the overall improvement in product quality and potential for faults to escape the inspection procedures. That a dramatic saving has been made in the cost of manufacturing this product, easily covering the costs of experimentation and new stencils, in a matter of days.
The experiments have shown that spending time working with x-ray images of placed components before reflow soldering allows improvements to be made to aperture design which can eliminate mid chip solder balling and spatter caused by excess solder paste or solder paste which is squeezed off pads by the placement of the component.
There is no simple redesign of all apertures which overcomes these issues but by using x-ray technology to examine what is happening under specific component types or in areas where solder balling is an issue, it is possible to make significant improvements to first time yields.
This is a very strong argument for increased effort at the front end of a job, an increase in engineering time will lead to significant improvements in yield and reduced cost of manufacturing. However, the real benefit may be in the longevity of the product as it does not undergo rework and repair which have been shown many times to have a detrimental effect on the products life cycle and sometimes also performance in the field.
Initially Published in the SMTA Proceedings
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