From: lasner@watsun.cc.columbia.edu (Charles Lasner)
Subject: Re: Flip-Chips (was Re: FAQ)
Reply-To: lasner@watsun.cc.columbia.edu (Charles Lasner)
Organization: Columbia University
Date: Mon, 5 Oct 1992 07:54:14 GMT

In article <1992Oct2.232759.21488@netcom.com> rbp@netcom.com (Bob Pasker) writes:
>
>one nit to pick: olson et al chose not to call their machines
>"computers" because the investors would have then refused to buy in.
>ostensibly, they felt that the cmputer field would be less lucrative
>than makeing modules, like the "flip-chip."  could be urban legend, 
>could be truth, but its at least plausible.  someone call ken and
>ask him.

The notion about PDP's versus computers sounds plausible, but does anyone
have any info about it?

In any case, the Flip-Chip notion is a legend in itself.  For some reason,
undue emphasis is given to a detail of the implementation of some module
series, mostly R-series, but S and probably B as well *might* have this.

Flip-Chips are the small hybrid packages that are sometimes used on some of the
cards which are used in the early models of PDP-8, -10, -9, LINC-8, etc. in
these three main module series.  They are *never* a requirement for most 
modules.  If you pickup a random R-series module, it has transistors and
diodes and resistors and capacitors on it, no Flip-Chips at all.  Some of
the cards do substitute a little thick-film hybrid package for some of
the diodes, resistors and capacitors, essentially the components that make
up the so-called DCD gate commonly used in the logic.  The purpose is to
standardize the signals between cards, and is essentially a way to get
a level and/or a pulse as input to a flip-flop or gate.  Today we would just
have edge and level-triggered flip-flops where necessary.  In any case, some
of the leads of the DCD gate are component interconnect, so it can be looked
upon as a module.  DEC eventually did make Flip-Chip modules, so on certain
cards, you find these little packages in lieu of the standard set of components
which doesn't dramatically cut down on either parts count or even lead count.
In fact, many of the Flip-Chips have proven to be unreliable and have to be
replaced with the normal complement of discrete components when repairing the
module.  All of them are readily available: silver-mica capacitors, 1/4 Watt
resistors and miniature silicon diodes are all that constitutes what's in a 
Flip-Chip.

Eventually, some "evil" modules appeared.  They usually have numbers like
R107 s1a where the s1a means that someone bothered to relayout the module so
it can *only* take Flip-Chips :-(.  (As opposed to the earlier design which
could take either the loose components or the same exact parts count as the
newer ones.  The only difference from a manufacturing standpoint is the
arbitrarily different etch and a modestly smaller drilled-hole count.  Hardly
worth offseting the cost of the redesign.)

For these modules, repair is more difficult.  In an pinch, the components can
be soldered together to make a Flip-Chip "equivalent" and then the whole bunch
is inserted into the Flip-Chip's solder holes.  Alternatively, a working
earlier card gets "cannabalized" for Flip-Chips to fix the Flip-Chip-only card.
The earlier design is then refitted with the loose components it is designed
to take.  When done, you have a working newer s1a card, and an older card
returned to older-still standards.

So, descriptions of clean rooms, etc. aside, Flip-Chips per se play a
secondary role in the production of modules sometimes described as "Flip-Chip
modules" which may be 100% devoid of Flip-Chips.  Moreover, some modules such
as R001 and R002 which only contain diodes, never have Flip-Chips.  Can anyone
post the suffix variation for some of these cards that indicate that the
card also lacks the diodes?  (Yes, I saw one of these once.  It wasn't s1a, but
some other suffix meaning all components deleted!)  To my knowledge, there
isn't a single R, S, or B-series module which *only* came in Flip-Chip-bearing
variety.  Moreover, Flip-Chips only came in a few standard configurations, 
since they only replaced certain component sets, thus cards tended to have
some discrete components anyway.  And they *never* replace the discrete
transistors used on the module.

cjl (in possession if literally thousands of R-series modules)
Newsgroups: alt.sys.pdp8,alt.folklore.computers,misc.forsale.computers