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From:
Stephen Gregory <[log in to unmask]>
Reply To:
TechNet E-Mail Forum <[log in to unmask]>, Stephen Gregory <[log in to unmask]>
Date:
Thu, 17 Nov 2005 07:37:07 -0600
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Mornin' Inge!

I have the thesis posted. Go to:

http://www.stevezeva.homestead.com/files/Ericsson_Thesis.pdf

I have a question; what's Marmite?

-Steve Gregory-

-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Ingemar Hernefjord
Sent: Thursday, November 17, 2005 2:06 AM
To: [log in to unmask]
Subject: [TN] SV: [TN] SV: [TN] [LF] Reliability prediction of LF soldering

Werner & TNs,
More about highly qualified input. We made a FEM model for a simple 20
ball BGA for computing in Nastran or Matlab. Despite the small package,
the number of bits became immense:  The model consists of 12611 elements
with a total of 13793 nodes. The problem gives 40179 unknown nodal
displacements.
I'm an amateur on this, but I realise the complexity if one has to make
a model of our superBGAs (5,000 balls) and the rest of the components on
the board (hundreds). It seems to me as a very expensive and
timeconsuming issue. You can select simpler models and fewer elements,
of course, but at the cost of confidence. The report I refer to does not
give MTBF figures but is a study before diving into the nighmare of
verifying our customer's demand on MTBF. The study ought to convince
anyone about the complexity of solder joint calculations, far from a
piece of a cake. I'm deeply impressed by the guys who work with these
tools! The report is of totally no interest for  most of you, but casts
some light on said complexity for some of the TNs. For Werner, this is
babyfood, of course, for me like eating a full can of Marmite. Steve,
I'm sending the report to you soon. 64 pages!

If someone wants to stop my endless discussion, plz tell me, I don't
want to get bad reputation in this fine forum. Just say 'shut up old
man, enough! '
Inge


-----Ursprungligt meddelande-----
Från: TechNet [mailto:[log in to unmask]] För Ingemar Hernefjord
Skickat: den 16 november 2005 23:00
Till: [log in to unmask]
Ämne: [TN] SV: [TN] [LF] Reliability prediction of LF soldering

One of many then, note what I've bolded. It seems not to be an easy task
to predict MTBF for a big BGA, because a lot is estimations, and this
'estimator' can't be anyone else than a experienced specialist on
soldering, and not all have such gurus within the company. About
Handbook 217, the guys here say that most figures in the tables are
based on a decade or more back, and myself, I have not found any useful
information on solder joints. So, you need a guru like Werner at the
company (fortunately we have one or more..Werner, you can not any longer
be the only guru at this planet...he-he)
To make it easier for you to find, I've marked the section Note1 and
Note2 below.
 
The sentence is that MTBF calculations demand highly qualified input,
but also, with that fulfilled, it's the best instrument we have. Without
the qualified input, it may be a misleading instrument.
 
Maybe it's not too  late for me to learn...
 
Inge
 
 
 
 
 ---------------------------------
--------------------------------------------------------
 
FAILURE ANALYSIS
 
MTBF Revisited
A Tutorial
 
by David A. Case, Aironet Wireless Communications
 
Almost everyone in the electronics industry is familiar with the term
mean time between failures (MTBF). Quite often, the term is
misinterpreted and misapplied. This is especially true when products are
delivered, failure reports begin to arrive, and MTBF predictions are not
validated by tabulations of real-world trouble reports.
 
Were the MTBF calculations wrong? What happened? To understand the
prediction of reliability, we need to examine these frequently asked
questions.
 
Defining Reliability
 
Before we consider reliability predictions, let's look at the meaning of
reliability. The generally accepted definition of reliability is the
probability that a device will provide adequate operation for a given
time in its intended application. This involves two judgment questions:
 
What is adequate operation? 
What is the intended application? 
Keep in mind the answers to these questions as we look at the part of
that definition that can be measured: What is the best estimate of the
probable MTBF?
 
Back to the judgment items. If your car radio has suitable AM reception
but fails to receive FM stations, is the entire car unreliable? Or is
this an inappropriate criterion for making such a sweeping judgment? And
if you drive through two feet of water and the car stops, is the car
unreliable, or is such treatment outside the bounds of its intended
application? The reliability specialist must sort out these types of
questions before proceeding to MTBF calculations and define the answers
as part of a reliability prediction.
 
Predicting MTBF Through Calculations
 
Two widely accepted standards can be used to calculate MTBF. Most
government programs prefer calculations per the latest version of
MIL-HDBK-217, while many commercial programs use the Bellcore method.1,2
The current government version is based on work begun many years ago by
the Reliability Analysis Center and Rome Laboratory at Griffiss Air
Force Base. The Bellcore version is a derivative of that handbook,
modified and simplified in 1985 by Bell Communications Research, now
Telcordia Technologies.
 
Each document contains failure-rate models for parts used in typical
electronic products, such as ICs, diodes, transistors, capacitors,
relays, switches, and connectors. The rates are based on the best
available data from actual applications. Several differences exist
between the two methods, one of the most obvious is the expression of
failures per 106 hours (MIL-HDBK-217) or failures per 109 hours
(Bellcore).
 
As an example of an MTBF calculation, consider a hypothetical product
with four parts. The estimated failure rates per 106 hours for those
parts, operating at a given temperature, are available from the
manufacturers. Adding the estimated rates, we get the estimated failure
rate for the total product. To determine the MTBF, we divide 106 by the
product failure rate, giving us the estimated mean number of hours
between failures (Table 1).
 
Though most MTBF predictions are based on a single product, a more
realistic way of expressing the result is based on 100 or 1,000
products. If we have a failure rate of 1.00 product per 106 hours, then
100 products will have a failure rate of 100 products per 106 hours. The
MTBF of 100 units then is projected as 10,000 hours.
 
At this point, some assumptions must be made and documented along with
the calculations:
 
Note 1:
 
The component parts are of uniform reliability, even though we know
there are differences. 
The parts count is correct, although the design may not be complete. 
The estimated failure rates for the four component parts are valid
rates, even though we know they are just estimates. 
The operating temperature on which we established the component failure
rates is correct for our application. 
Finally, we must define those two judgment issues we mentioned earlier:
adequate operation and intended application. All this will make the
prediction most meaningful and help fine-tune the prediction later if
our assumptions prove to be flawed.
 
There are two benefits of predicting MTBF on a product. First, it may be
a customer requirement, in which case the other benefit doesn't count.
Second, it can be done long before the design is committed to
production, giving a heads-up evaluation of the product. It even
highlights the weak points, so they can be improved at minimum expense.
 
Fortunately for the reliability specialist, software is available to
simplify reliability calculations.3 The computer allows you to select
stress levels such as operating voltage and temperature to simulate
real-world conditions that the product will encounter.
 
Note 2:
 
With all this, don't overrate the MTBF number. Sure, it is a very
precise calculation, but it is based on estimates. They are the best
estimates we can get, but there still is that lingering uncertainty.
 
Assessing Failure Rates Through Failure Reports
 
After products have been delivered and in service for a few months,
reality sets in. Possibly the failure reports show a significantly lower
or higher failure rate than your calculations predicted. If so, what
happened? Does this mean that your calculation of MTBF is a flawed
process? No. Or if the numbers match within a few points, does this mean
that you don't need to analyze field failure reports? Again, no. Both
methods of failure analysis are important, and there are reasons for any
significant differences.
 
Let's go back to the definition of reliability and the two judgment
questions:
 
What is the inadequate operation defined as a failure? Do the failure
reports for this product reflect your definition of adequacy? 
Was the application within the boundaries for which the product was
designed? Was the power input within limits? What about the operating
environment? Was the product subjected to electrostatic discharge (ESD)
after delivery? Was it dropped or mishandled in some other way to cause
the reported failure? 
Another factor that causes variations in the reliability prediction is
the inevitable difference in failure rates for component parts that look
the same and come from the same manufacturer. Remember, those failure
rates are based on averages, not absolutes.
 
Then you must consider your manufacturing practices. Did you make serial
number 10,000 the same as serial number 10? Each of these items can
modulate your failure rates.
 
What It All Means
 
The MTBF prediction, properly calculated and carefully footnoted with
the assumptions on which it is based, is an excellent engineering tool.
It evaluates your product even before the design is finished and
highlights problem areas that should get further attention. It is a
valuable asset to your potential customer, helping to evaluate suppliers
and designs before a contract is awarded and assisting in logistics
planning for support of the product in the field.
 
There also is a fiscal use of the MTBF calculation. You can predict the
probable difference in the cost of warranty repairs if part B is
substituted for part A and make value judgments with some confidence in
their validity.
 
When the real-world failure reports begin to arrive and it seems like
the MTBF estimates were wrong, that is a good time to look at those
judgment questions again. Are the definitions of failure really valid?
Is the product being used correctly? Don't blame the estimates. When
reliability is the issue, use every tool at your disposal. All of them
are valuable.
 
 
-----Ursprungligt meddelande-----
Från: TechNet [mailto:[log in to unmask]] För Werner Engelmaier
Skickat: den 16 november 2005 20:00
Till: [log in to unmask]
Ämne: Re: [TN] [LF] Reliability prediction of LF soldering
 
Hi Ingemar,
The one 'good' thing about this lead-free idiocy is that it has put a
lot of
people to work, produced all manner of research grants, and just look at
all
these thesis topics.
 
Werner
 
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