stevef@well.UUCP (Steven Robert Fordyce) (08/19/86)
I know this is an old subject, but I was a bit behind in my reading, and it
took a while for me to get the courage to write--this is my first posting.
Some people on this net have implied or stated that four-layer PC boards
are of higher "quality" in general than double-sided boards. This is
not true. I am an electrical engineer with some experience with both double
sided and multi-layer boards. If a board works, it works--there is no
reason to prefer four-layer boards over one or two layer boards.
As a designer I like four-layer boards because they allow greater density,
reduce EMI, and *tend* to reduce cross-talk and noise. Note that I said
tend to reduce; four-layer boards can have noise problems too. I find that
with two layer boards I have to sit with the layout person daily to see
that the board is laid out properly with adequate ground and power traces,
separation between potentially noise sensitive and noise generating traces,
and that the grounds for power, digital, and low level analog are kept
separate and are connected at only one place (of course I realize the last
one doesn't apply to memory expansion boards). With four-layer boards I
can, and do pay less attention to the layout of the board. After all,
power and ground, the inner two layers are as wide as they can be.
There are however important disadvantages to four-layer boards. They are
about twice as expensive, they are more difficult to rework (you can get to
the +5v and ground planes), and there is the potential for shorts between
the inner layers and feed-throughs. On one batch of boards we received
there was a intermittent short between the ground plane and one trace.
These were tested boards and the problem didn't show up until after the
assembly was tested and in use. I don't want to over state this sort of
problem--it is probably rare, and shorts can occur on a two layer board,
but it is less likely and much less difficult to deal with.
On the last board I had a hand in, the way we justified the expense of a
four-layer board was that that alone was enough to get us past the FCC
without further shielding. That saved us money over all, but if that
hadn't been true we never would have gone to the trouble of a four-layer
board. And even so, I did have a problem with cross-talk, and ringing that
took a second rev of the board to correct. So regardless of the number of
layers time and care must be taken do a proper layout. And once that has
been done, cross-talk and ringing also won't be a problem irrespective of
the number of layers.
As a consumer I see no reason to prefer four-layer boards. The Amiga
itself laid out on a two layer board. Would it be in any sense better if
it was a four layer board? Would it be faster, use less power, last
longer, or be more reliable? The answer to all of these is no. At higher
frequencies four-layer become necessary to stop excessive ringing in
switching, but the Amiga is safely below where it becomes necessary.
Steve Fordyce
hplabs!well!stevefperry@well.UUCP (Perry S. Kivolowitz) (08/20/86)
* line fodder * Thanks to Steve Fordyce for his article. As Dennis, the middle ages socialist (from Monty Python's Holy Grail), said: That's What I'm On About. You said yourself that you, because you are a conscientious engineer, would spend a lot of time with a board layout person if your were producing a two layer board but would need to spend less time overseeing the layout person if you were producing a four layer board. This is because two layer boards are more easily screwed up (with respect to noise problems). Well, you'd spend the extra time, Steve, but others may not. I can't vouch for all manufacturer's of Amiga expansion hardware but it is clear that some people do not share the same commitment to quality as you (and I) have. Hey, when you care enough to send the very best...you'll go the extra distance to produce a four layer board. Access Associates (the Alegra) went the extra distance. ASDG (the Convertible .5, 1 and 2M) bit the bullet on added cost. And didn't someone say Comspec did as well (my long term memory is off-line). Perry S. Kivolowitz
grr@cbmvax.cbm.UUCP (George Robbins) (09/01/86)
Keywords: In article <1655@well.UUCP> perry@well.UUCP (Perry S. Kivolowitz) writes: > >You said yourself that you, because you are a conscientious engineer, would >spend a lot of time with a board layout person if your were producing a two >layer board but would need to spend less time overseeing the layout person >if you were producing a four layer board. > >Well, you'd spend the extra time, Steve, but others may not. I can't vouch >for all manufacturer's of Amiga expansion hardware but it is clear that some >people do not share the same commitment to quality as you (and I) have. > >Hey, when you care enough to send the very best...you'll go the extra distance >to produce a four layer board. > >Perry S. Kivolowitz Perry, this is still a bit misleading... An engineer desigining a PC board will use a multi-layer board because he feels that he has to, not because it yields some ineffably better product. 4 layer Benefits: automatic 'good' power and ground distibution simplified signal routing lower signal noise levels lower radiated noise levels (FCC!) more likely to work well on the first try more likely to pass FCC on the first try 4 layer Detriments: higher artwork and setup charges higher board cost (1.5-3 X) The real kicker is that the multi-layer board in the prototype or early production unit you look at is likely to be quietly replaced by a garden variety double-sided board if the vendor makes it into volume production. That extra cost of materials translates into either decreased profit margin or a non-competitive price. These are of course business world no-no's. -- George Robbins - now working with, uucp: {ihnp4|seismo|caip}!cbmvax!grr but no way officially representing arpa: cbmvax!grr@seismo.css.GOV Commodore, Engineering Department fone: 215-431-9255 (only by moonlite)