At 06:52 PM 8/27/98 +0200, Matthias Mansfeld wrote:
>My question is: Are there any good thumb rules for a good estimate in
>advance how long it takes to make a layout for a given schematic/
>partlist/ mechanical constraints drawing and how many layers are
>necessary? I get usually a schematic and mostly a partlist and with
>these informations I must make a good estimate for my customer how
>much time it will take and (depends on time indirectly) how much cash
>I want. The customers want the estimate ASAP, that means, I have not
>much time to check out placement/routing density, the necessary number
>of layers etc. And later it ends up with two layers more than the
>customer wants to pay for and a lot of unplanned, unpaid night
>sessions on my layout PC to get it ready in time or only few days
>late.
Good estimating is a core skill, important even if one is a captive
designer, but crucial if one is independent.
The first thing to nail down is one's own personal rule: *always* estimate
density before issuing any kind of price or time quote, unless density is
so low that it is not an issue. If one has this rule, and breaks it, then
one knows full well who is responsible for those "unplanned, unpaid nights."
For many years, I have followed this method of estimating density:
I convert a net list into a database of the parts involved in a board, and
reduce this to a list of the footprints used and the quantities for each. I
then enter the pin counts associated with each footprint, and the area that
the footprint takes. For through-hole design, this is pretty simple: the
area of one component is the rectangular space that would be assigned to
each component if the components were packed as close as is practical with
no regard for routing considerations: only minimum spacing betweeen pads
and body clearances are important.
Then the total pin count for the board is calculated, and also the total
area for the components. The latter is then divided by the total usable
area on the board. This ratio is a percentage. One then, by experience,
determines the difficulty for a certain percentage. And the difficulty may
be nudged by other considerations. It takes time to build new components,
for example. And an array of components that are to be bussed together
might be easily routed even at 100% density.
Anyway, I used to consider any two-sided board with over 50% density as
"penalty density." And for four layers, the figure was 60% density. Above
75 or 80 percent, even at four layers, unless the components pack well and
route easily, it can get quite difficult. Analog design, with lots of
discretes, can often pack pretty tightly and route easily, unless the
components vary wildly in size.
Now, I can look at what I just wrote and pick it full of holes. Exceptions,
exceptions. But until one starts quantifying density in some readily
cognisable way, there is nothing for one's intuition to chew upon. Do it.
I've been doing it for twenty years and I have yet to seriously underquote
an original design after doing a density study. But if anyone can come up
with a way to quote *changes*, let me know!
(I tend to requote the entire design and, unless the changes are minor,
tell the customer that the revised board will not cost more than an
original design; then I work by the hour. The only time I have been
seriously burned on a job was when I quoted a fixed price for a major change.)
Note that if one quotes well and accurately, one may sometimes lose jobs to
those who underbid. Be grateful.
An underbid job is often delivered late and with errors, and some of the
customers I have lost this way have come back with renewed loyalty....
[log in to unmask]
Abdulrahman Lomax
P.O. Box 423
Sonoma, CA 95476
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