INSTRUMENTATION by Ralph H. Müller
Failure Analysis Is a Continuing Challenge to the M a t h e m a t i c i a n , Physicist, Metallurgist, a n d the Analytical Chemist
or systems failures have M ATERIAL been with us since a n t i q u i t y . Icarus came to grief by flying too close to the sun, which p r o m p t l y melted his wax wings (poor choice of m a t e r i a l s ) . D a r i u s Green in his flying machine m a d e a crash landing because he used an iron kettle for a helmet (over-design in t h e safety d e p a r t m e n t ) . F o r an integrated sum of calamities—"For want of a nail a shoe was lost . . . " \vas a record for its time. T h e problem has reached staggering proportions in this space age. I t reaches the point of a b s u r d i t y when t h e launching of a satellite or rocket costing m a n y millions of dollars, h a s t o be scratched because a twenty-cent c a pacitor blew out. An enormous a m o u n t of s t u d y and money is being devoted t o this question. F o r example, the Air Force systems reliability p r o g r a m now costs $30 million. T h e importance of failures and rejects varies with t h e particular p r o b lem. In a n a u t o m a t i c production line, rejects are identified somewhere during the operation a n d if these c a n be salvaged b y remelting or refabrication the economic losses can be held t o a reasonable level. W h e n one is dealing with a system involving h u m a n beings, a far greater degree of reliability and built-in safeguards becomes m a n d a t o r y . Again, in launching satellites a v e r y large fraction of the total cost m u s t
go into exhaustive checks, inspection, and rigid control of tens of thousands of components. A few m o n t h s ago A r m o u r Research F o u n d a t i o n held its first symposium on the physics of failure in electronics [Electronics 35, X o . 4 1 , 28 ( 1 9 6 2 ) ] . Some quotes from this brief b u t illuminating report illustrate t h e current ideas on the subject. C. H . Zierdt, of General Electric, questioned whether it was economically possible to take precautions against all the h u n d r e d s of possible failure causes. H e described a diagnostic m e t h o d used in failure analysis and process control for the Alinuteman 703 transistor, t h a t reduced 30 possible failure modes t o 6, and reduced total mechanisms to 11. According t o R o b e r t K i r k m a n of Space Technology Labs, statistical a p proaches t o failure have already told about as much as they can and more precise definitions—including such generally overlooked factors as h u m a n errors, foreign inclusions a n d loose p a r t s — a r e needed. C u r r e n t noise m e a s u r e m e n t s as a failure analysis tool for film resistors were discussed by J o h n Curtis of Corning Glass W o r k s . Units exhibiting unusually high noise show defect? visible under a microscope. A discrete noise level is useful t o distinguish unacceptable resistors from normals. Although not all noisy resistors are poor
performers, all resistors tending t o b e abnormal are noisy. C u r r e n t noise refers to the root m e a n square fluctuating voltage appearing across a resistor when it is carrying a definite current. This is superimposed upon the noise voltage which appears across a resistor carrying n o current. T h e l a t t e r is a consequence of t h e thermal agitation of electrons in t h e conductor. W i t h the most meticulous a t t e n t i o n devoted t o testing, analyzing a n d quality control, there is always a gap between t h e high probability of good performance a n d absolute certainty. By testing h u n d r e d s of thousands of units of a given t y p e , one can arrive a t a statistical estimate of the reliability of any single unit which shall be used but this does little more t h a n settle the betting odds. If t h e chosen unit is subjected t o stresses close to destructive limit, then one h a s probably shortened its useful life. F o r a long time the analyst has m a d e i m p o r t a n t contributions to quality control. H e can make still greater contributions in reliability tests because the general problem is one requiring t h e skills of a dozen different disciplines. I t is evident from the m a g n i t u d e of t h e problem t h a t his contributions will have to be in the n a t u r e of nondestructive tests, semi or fully a u t o m a t i c , a n d capable of being conducted rapidly. VOL. 34, NO. 13, DECEMBER 1962
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