At 09:21 AM 7/9/01 -0700, Brad Velander wrote:
>         I read your comments and went to my IPC-2222. I am no IPC expert by
>any stretch but aren't the tables you specify (4-2 ... 4-6) only for copper
>clad laminates, where are the prepregs? Or is this a typical IPC f***up and
>they have mislabeled their tables to imply something which was not intended,
>I hate their documents because of these types of incongruities.
>         Can anybody answer these questions in regards to these tables?
>What is DS?
>Where is the X & Y CTEs? Why are they not included?
>What exactly is CHEM?
>What do the +, - and blanks mean in the last three columns?
>Damn the IPC is so stupid when it comes to their documents, you need a guide
>to their guides because they don't adequately support their information.

I'm not convinced that the IPC specifications were put together by the best 
and brightest in our field, but let's assume that it was. Unfortunately, 
being the best and brightest designer or engineer does not make one the 
best and brightest technical writer.

As to a guide to the guides, they will also sell you that. (The IPC PWB 
Designer Certification Study Guide). Unfortunately -- I've been using that 
word a lot today, haven't I? -- it does not cover the questions Mr. 
Velander asked.

However, if you read the specification several times, you might notice that 
4.3.6 explains that the tables "provide information on the properties of 
finished bare laminates for different prepreg constructions. To establish 
final laminate thickness with copper, add 35 um for each oz. of copper on 
the laminate."

I could guess at what some of the abbreviations mean, but it would be just 
that: a guess.

Here are my guesses:

DS: Dielectric Strength (this would be measured in volts/mm, for example)
Or it might mean Dimensional Stability.

Z CTE: Coefficient of Thermal Expansion in the Z axis (i.e., thickness). I 
don't know why the X and Y CTEs are not given.

By the way, looking into this made me realize that the tables cover 
laminate, not prepreg. There are other specs for prepreg, IPC-L-109 and 
IPC-4101. Full employment for IPC staff!

+ means "better," and - means "worse," and a blank is in between. It's an 
intelligence test. As a consolation, it took me about an hour to figure 
this out. There is a little + after the Better label on the symbol chart.

What DRILL means in this chart I find obscure.

I've managed to avoid, in my career, highly-specified boards for military 
or other use; other boards have simply referred to the specification, and 
the engineer did not understand the specification any more than anyone 
else. I think the idea is that if the board turns out to have a problem, 
and you just made 10,000 of them, you could do the research to find out 
what the spec means and thus if the fabricator violated it. But if one has 
specified the prepreg material, that won't work, since the fabricator will 
have followed specific instructions, and the chart was merely a design 
guide, not a specification, per se.

In other words, the IPC has mixed a design guide with a specification. They 
are different animals and should not be combined. They have done this all 
over the place. It makes the documents hard to read as design guides and 
even harder to read as specifications.

Sorry, mention the IPC and I tend to get a little steamed. They are living 
in a world about twenty years old, before the internet.

I asked them why the CD version of the design certification kit was more 
expensive than the printed version. I was told that it was because they had 
a warehouse full of the printed versions and they needed to get rid of 
them. Way to go!

Not only does this mean that they made a serious error in their print run, 
but they also apparently never heard of print-on-demand, which for 
specifications is most appropriate. Otherwise you end up delaying the next 
rev until that warehouse starts to look empty.


I was a member of the Designer's Council, it recently lapsed and I haven't 
renewed yet. The Designer's Council is not responsible to the members, it 
is a top-down organization. You can find the list of the Board of Directors 

The site is not specific about this, but from that fact that I never heard 
of anything like elections, as a member-at-large, I would assume that it is 
a typical non-profit with a self-electing board; that is, the board elects 
its successor members. They might pay attention to local council 
recommendations, they might not.

Such organizations tend to become turgid bureaucracies in spite of the best 
intentions. It can be just as bad when the members can vote, if the 
organizations are very large and the members not highly motivated to vote, 
and proxy voting is allowed, and the board has privileged access to the 
members for the issuance of proxies. An example is the California State 
Automobile Association, which is largely a device for selling insurance. 
Legally, I think, it is a nonprofit organization and thus tax-exempt on 
some of its activities, but there is nothing that prevents such an 
organization from paying "reasonable" salaries to its officers, and 
reasonable can be all over the map.

Abdulrahman Lomax
P.O. Box 690
El Verano, CA 95433

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