Quality improvement program addressed to M.O.S. microprocessors

Microelectro, Reliah.. Vol. 22. No. 2. pp. 207 216. 1982.

0026-2714/82/020207-10503.00/0 © 1982 Pergamon Press Ltd.

Printed in Great Britain.

QUALITY

IMPROVEMENT IVLO.S.

PROGRAM

ADDRESSED

TO

MICROPROCESSORS

CHARLES

L. HUTCHINS and

B. J. PAICIUS

Texas Instruments Incorporated, HOUSTON, Texas, U.S.A., (Received for publication 9th April 1981) The advantages of a microprocessor application enhanced system can be classified into three broad categories: engineering system advantages, human factors advantages, and economic advantages. Each class can show an advantage over the approach of the previous generation. These advantages converge in an improvement in the realm of product quality and reliability.

At the e n g i n e e r i n g or system level, a d i s c i p l i n e is enforced in the transition from the older system approach to t h e n e w . This discipline requires the engineer and manager to critically review and decide on specific system goals, r e q u i r e d function, desired function, u n n e c e s s a r y function, and on occasion, the regressive function. In this way the entire system is evaluated in terms of: What does our system do? What should our system do today? What should our system be in the future? This goal i d e n t i f i c a t i o n process requires of the e n g i n e e r and m a n a g e r that they identify the system functions, specify o p e r a t i n g conditions, and quantify the system a d v a n t a g e s in monetary terms. Further, the f e a s i b i l i t y study of a microp r o c e s s o r enhanced a p p l i c a t i o n should provide clarity of purpose and accuracy of intent. These are the m a j o r system advantages. In contrast, the human factors analysis c o n s i d e r s the same s y s t e m factors from an i n t e r f a c e / s u p p o r t approach. For a typical m i c r o p r o c e s s o r a p p l i c a t i o n enhanced system, the m a n p o w e r required to supply the p r e v i o u s c o m p a r a b l e function or service is reduced allowing a b e t t e r u t i l i z a t i o n of precious resources. This is u s u a l l y a c c o m p a n i e d by an increased system q u a l i t y and r e l i a b i l i t y through p r e d e t e r m i n e d logic execution locked into the function of the m i c r o p r o c e s s o r . As an example, consider the decision of Texas Instruments m a n a g e m e n t to provide all p r o f e s s i o n a l s in e n g i n e e r i n g with s c i e n t i f i c p r o g r a m m a b l e TI-59 calculators. U t i l i z a t i o n of the c a l c u l a t o r provided a reduction in time share demand for the company computer users. C a l c u l a t i o n s which did not require the special computer features could be done anywhere with no time share delay. The result was to i m p r o v e the people and asset e f f e c t i v e n e s s while i m p r o v i n g the TI-59 207

C, L. HUTCHINSand B. J. PAICIUS

208

product through new application oriented programs generated internally. Today, some four thousand programs are on file in the PPX or professional program exchange on many diverse topics. In this way, the predecided logic can be developed and built-in which prevents our people from reinventing the wheel. These are the major human factors benefits. Thirdly, the economic advantages of microprocessor enhanced application systems can be briefly shown. In addition to the aforementioned system streamlining from the analysis of function, the MOS Microprocessor continues to develop more and more functions in ever increasing speed with improved reliability. Here is why MOS makes sense: In the short thirty years since the invention of the transistor (the first semiconductor device to exhibit amplification), there have been more inventions and more scientific and engineering accomplishments than in all time previous.l The field of digital electronics (especially computers) has been the greatest contributor of new products for these accomplishments and, therefore, has become one of the most rapidly growing industries. Manufacturers of semiconductor components (transistors, integrated circuits, microprocessors and memories) have been providin.g the building blocks, and the equipment manufacturers have been taking advantage of the opportunity by developing the most sophisticated systems that are economically feasible. The cost of the hardware components for a typical digital system has been decreasing with time because new and more powerful semiconductor devices have been developed. Equally important is the fact that the development cost for the typical digital system hardware has also been decreasing. Figure 1 illustrates how impressive this cost reduction has been.

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Contrast the figures of 7-8 million dollars in the early fifties with 8-9 thousand dollars in the late seventies; digital system development cost has been reduced by a factor of one thousand in a period of 25 years! An extension of this trend indicates that typical system hardware development cost will be approximately $I,000 by 1985. How has this been accomplished? Figure 2 shows what has been happening. As the number of components per chip of silicon increases, the development cost for each chip also increases. For a semiconductor manufacturer, volume production is required to offset the development cost. Semiconductor devices are therefore being batch fabricated - a few hundred, a few thousand per chip - and this means lower cost per active element group of AEG. An AEG is defined as a logic gate, flip-flop, or a memory cell. With this breakthrough in the concept of LSI application, the semiconductor technologists have been motivated to continue to increase the number of AEG's per device. Figure 3 projects the growth of AEG's per chip to over 106 by 1985 - a level sufficient for a single chip 32-bit microcomputer. The 16-bit MOS Microprocessor and 16K MOS RAM require about 50,000 AEG's. MOS RAM's of 64K bits requiring up to I00,000 AEG's are not unrealistic extensions of the trends; they are real products rapidly moving into the marketplace. New advances are being made in MOS process technology to achieve the packing densities needed for the future. As Figure 3 indicates, optical techniques for defining regions and interconnections reach a resolution limit at about 10 s AEG's. E-beam and X-ray technology will be required to further increase component density. ...Now with this understanding of the systems advantages of an MOS Microprocessor system the areas of quality improvement and reliability can be developed along similar lines. The first of these concepts in quality improvement may be called "driving the learning curve".

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DRIVING THE LEARNING CURVE

Learning curve theory states that learning is a log function reduction in cost versus production volume. 2 This decline goes on in time without limit (in constant monetary value). The rate of decline is surprisingly consistent, even from industry to industry. This is generally shown by Figure 4. However, these observed or inferred reductions in cost as volume increases are not necessarily automatic. They depend crucially upon a competent management that seeks ways to obtain cost reduction as volume expands. To this extent the relationship is of normal potential rather than one of certainty. The provides quality, material, flow.

same is true with quality. The increasing volume an opportunity for learning associated with product i.e. improving processes, converting to improved and minimizing problem points in the manufacturing

For example, the TMS I000 was the first w i d e s p r e a d app l i c a t i o n of a f o u r - b i t m i c r o c o m p u t e r . The e a r l y f a i l u r e rates that w e r e a c h i e v e d in the first year of p r o d u c t i o n exceeded 1.3%. T h e n gradual and p a i n s t a k i n g a n a l y s i s of the p r o b l e m s w h i c h l i m i t e d the product quality w e r e sought and e l i m i n a t e d or reduced. T h r o u g h this p r o c e s s of m o v i n g down the l e a r n i n g curve, the failure rate d e c r e a s e d until today it r e m a i n s in the area less than 0.08% ( F i g u r e 5). Thus, the T M S i000 e x e m p l i f i e s the learning curve theory.

2 "Perspectives on Experience", The Boston Consulting Group, 1968, Chapter l , p . l .

M.O.S. microprocessors

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H o w e v e r , in r e s p o n s e to the A u t o m o t i v e and o t h e r h i g h q u a l i t y / r e l i a b i l i t y m a r k e t s where cost is also a factor, time w o u l d not a l l o w the usual learning curve d e v e l o p m e n t . A u t o m o t i v e p r o d u c t s required a m i l i t a r y type p e r f o r m a n c e but w i t h a c o n s u m e r market type supply and cost. Further, with an a l r e a d y g r o w i n g a t m o s p h e r e of legal l i a b d l i t y for consumer p r o t e c t i o n , the devices w o u l d have to be r e l i a b l e as well. The decision was made to use one product, like the TMS I000, and use the knowledge of this program to design and engineer as many problems as possible out of the program at the onset. This included the use of standard proven piece parts where it was permitted by design, as well as using processes already existing which were known to be dependable. Next, a quality and reliability testing and evaluation procedure was rigorously followed to identify problem areas and verify the effectiveness of results. T h r o u g h o u t this phase the s t r a t e g i e s of t e s t i n g and opt i m i z a t i o n w e r e d e v e l o p e d to achieve a h i g h l y r e l i a b l e device on the n o r m a l p r o d u c t i o n lines. Special e l e c t r i c a l test programs w e r e w r i t t e n to allow the a s s e m b l y plant to o b t a i n rapid f e e d b a c k on the quality of their m a n u f a c t u r i n g work. New e n g i n e e r s w e r e u t i l i z e d as p r o d u c t i o n e x p e d i t e r s allowing them to u n d e r s t a n d h o w their parts w e r e built w h i l e insuring smooth p r o d u c t i o n flow. P r o g r a m s like these and o t h e r s allowed for e n g i n e e r i n g and system a d v a n t a g e s over the TMS lO00 program.

212

C . L . HUTCHINS and B. J. PAICIUS

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Next, training and awareness programs were implemented to keep people in all areas aware of the required product quality levels. The initial responses indicated that much work would be required in maintaining control in process and production areas where volume of production had typically spoken louder than needed quality. However, the effort paid off in relatively short time and the human factors variables were controlled. Finally, the economic advantages were carefully monitored to insure system goals were being met. The kind of improvement seen is an initial yield and corresponding quality level significantly above the norm. This compares favorably considering that the technology was of the newer N-Channel MOS compared with the older P-Channel technology of the TMS I000. The higher yield provided for better market penetration and higher volumes, which in turn provided a better opportunity for learning curve development. QUALITY

IMPROVEMENT

PROCEDURE

T h r o u g h o u t the A u t o m o t i v e e x p e r i e n c e , the c o m m i t m e n t w a s e a r l y e s t a b l i s h e d to e v a l u a t e p r o b l e m s as soon as t h e y p r e s e n t e d t h e m s e l v e s t h r o u g h a p r o c e d u r e o b t a i n e d f r o m one of o u r c u s t o m e r s . ~ T h e t e c h n i q u e is not u n i q u e , ' b u t the case h a s b e e n m a d e for the f o l l o w - t h r o u g h . The s e v e n f o l d a p p r o a c h is s u m m a r i z e d in F i g u r e 6. T h i s p r o c e d u r e has b e e n s t a t e d in m a n y forms and can b e e f f e c t i v e in all if f o l l o w e d in a d i s c i p l i n e d m a n n e r . It can b e v e r y e a s y to s k i p s t e p s or c o m p l e t e only the f i r s t three. To e x e c u t e o n l y a p a r t of t h e p r o c e d u r e w o u l d p r o d u c e p a r t i a l r e s u l t s , p o s s i b l y a l l o w i n g the p r o b l e m to r e c u r later. T h u s , no real l e a r n i n g t a k e s p l a c e and one is d o o m e d to r e p e a t h i s " e r r o r s " .

M.O.S. microprocessors For example, o n e of t h e s y s t e m s u p p o r t c h i p s s h o w e d n o p r o b l e m in m e e t i n g t h e e a r l y t e m p c y c l e c r i t e r i a . This plastic device passed the initial qualification requirement of -30/I05°C t e m p c y c l e at I 0 0 0 c y c l e s , Subsequently, the de_QUALITY IMPROVEMENT PROCEDURE

THE PROBLEM THE PROBLEM MUST BE DESCRIBED AND QUANTIFIED IN EX~DLICIT TERMS. THE CAUSE THE CAUSE MUST BE IDENTIFIED AND DEFINED AS ACCURATELY AS POSSIBLE BASED UPON THE AVAILABLE INFORMATION. SHORT TERM ACTION IDENTIFY THE IMMEDIATE ACTION TAKEN TO LIMIT THE PROBLEM AND THE EFFECTIVE DATE OF ACTION. LONG RANGE ACTION SHOW THE PLANNED ACTION AND EFFECTIVE DATE FOR PERMANENT PROBLEM CORRECTION. VERIFICATION MEASURE THE EFFECTIVENESS OF ACTIONS BY INDICATORS %~ICB CHART PROBLEM SEVERITY. ACTIONS ARE NOT CONSIDERED COMPLETE UNTIL INDICATORS SHOW EFFECTIVE RESOLUTION. OUTLOOK PROJECT THE LONG RANGE EFFECT OF THE PROBLEM Ah~ CORRECTIVE ACTIONS. STATE THE DEGREE OF CONFIDENCE IN THE EFFECTIVENESS OF THE ACTIONS. COST ACCOUNT FOR THE TOTAL COST OF CORRECTIONS.

Figure 6

v i c e s h o w e d f a i l u r e s in t h e l a t t e r p a r t o f t h e p r o d u c t i o n . The problem was identified as an internal c r a c k i n g as a r e sult of thermal f a t i g u e in t h e m o l d c o m p o u n d . The immediate action was to investigate the compound to evaluate the possibility of a change from the original lot. A barely perceptible change was detected. A task force was put together for evaluation of this type of problem and the packaging considerations were reviewed. The hypothesis that the thermal expansion characteristic mismatch between the mold compound filler system and the silicon die was the base cause for the cracking was tested and verified. The result was a modification to the plastic mold compound which allowed over 1000 cycles of a -65/150 ° temperature cycle performance without a failure. This modification was implemented with negligible cost. The outlook was encouraging with the experience gained plus the overall product improvement. This corrective a c t i o n l o o p is a n o t h e r o f t h e m e c h a n i s m s for driving the quality improvement cycle toward meeting even greater markets with superior products that constantly improve. Engaging in i n c r e a s i n g markets with stricter and stricter demands creates these enhanced products from the standard production line by improving the understanding of t h e p r o d u c t a s w e l l as u p g r a d i n g the standard product. This r e s u l t s in o v e r a l l quality improvement.

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C.L. HUTCHINS and B. J. PAICIUS

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In following the quality improvement procedure given above, considerable emphasis was placed on prompt and thorough failure analysis. This was accomplished via the flow chart shown in Figure 7. Following this produced the information to completely answer the problem question and in' some cases provided help in defining the cause. In the remaining cases, consultations with various MOS manufacturing specialists provided details on the more subtle causes and concurrently recommended short term and long term corrective actions.

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V e r i f i c a t i o n of these c o r r e c t i v e a c t i o n s was also formalized. A m a t r i x as shown in F i g u r e 8 was used as a g u i d e l i n e to d e t e r m i n e w h i c h environmental, m e c h a n S c a l , or e l e c t r i c a l test w o u l d be used. Care must be e x e r c i s e d here to assure that r e p r e s e n t a t i v e samples of the c o r r e c t i v e action are selected and are c o m p a r e d against o r i g i n a l s a m p l e s u n d e r controlled c o n d i t i o n s . A l t h o u g h " b u i l t - i n " quality is the best and t h e r e f o r e the p r e f e r r e d method, a d d i t i o n a l steps for b u r n - i n and m o r e thorough t e s t i n g have also been p r o v e d b e n e f i c i a l . In the case of this p r o g r a m both "infant m o r t a l i t y " and m a r g i n a l

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electrical u n i t s w e r e found to be a m e a s u r e a b l e p r o b l e m at incoming and d u r i n g equipment m a n u f a c t u r i n g . To r e d u c e the impact of these, the i n t e g r a t e d circuit m a n u f a c t u r i n g flow was m o d i f i e d as shown in F i g u r e 9. The s i g n i f i c a n t a d d i t i o n a l steps are 1 0 0 % b u r n - i n under p s u e d o - o p e r a t i o n a l v o l t a g e s and a two-pass test (high temp followed by room temp) a f t e r burn-in. T h e y i e l d loss a p p r o x i m a t e d the early loss at the c u s t o m e r ' s i n c o m i n g and assembly line, and the f a i l u r e m o d e s were also similar. SUMMARY The net result of this p r o g r a m r e s u l t e d in the first generation of a u t o m o t i v e m i c r o p r o c e s s o r s w h i c h a c h i e v e d a 0.095~ f a i l u r e r a t e in the first year of p r o d u c t i o n , c o m p a r e d with the e a r l y > 1 . 3 ~ rate for the first year of the TMS i000.

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