Help: ground plane under microcontroller.

[Spehro Pefhany]

HAH!!! Joerg, you hit the nail on the head. This customer fully
believes that any other color circuit board besides BLUE will not work.
So, I agreed to supply blue circuit boards instead of green.

Two possibilities.

1) The guy really doesn't know what's going on, and is taking advice
from someone who doesn't know either.

2) The guy has already selected a competitive product (because he gets
a better kickback or his brother works there or whatever) and is
looking for a reason to give the work to them instead of you. The
blue board was his first attempt, and when you got around that, he's
trying this. This could backfire, but you could go to him and say
you think you've worked out a way to make the ground plane change
in time, and ask if there's anything else. If another goofy reason
pops up, politely run the other way.

OTOH, for thirty megadollars, you can get creative. Tell him you are
going to paint the bottom of the micro with a metallic paint to serve as
a ground plane, and add a contact for it to the board, or run a line of
paint over to the ground lead of the micro, or whatever. After the
board is stuffed, have somebody in production use a paint pen to draw a
silver line on the bottom outside edge of the micro - "overspray" from
when the bottom was coated.

Matt Roberds[/QUOTE]

One "tough EMC environment" product I was involved with even had a
stamped metal shield "ground plane" over the *top* of the QFP package.


Best regards,
Spehro Pefhany

 

[Paul Burke]

I have a new customer who, for some unknown reason, is critical of my
design. His only problem with it is the fact that there is not a
continuous copper ground fill under the micro in the top layer of the
board. Traces and vias under the part break it up.

Are you alone in selling into this app, or is there another firm
involved? Have they got in first, with their only selling point being
the solid toplayer ground plane, plus a course of educational visits to
expensive restaurants and golf clubs?

Paul Burke

 

[PeteS]

As an extra motivation for your customer (and for you)

Apart from the fact that *for this type of unit*, such an extra plane
is not necessary for performance or EMC (having already passed), that
such a change to the artwork would require you to re-do EMC (well, it
would in Europe, anyway) taking extra time and costing extra money.

Find out if he's willing to pay for the extra costs involved and the
extra time involved.

I have, unfortunately, had an ME who got a bee in his bonnet about 'the
right way' to do some things, and believed only that way was right.
That led to some fights until I delivered 5 designs in a row with no
errors (which surprised me as much as anyone

Cheers

PeteS

 

[John Woodgate]

I read in sci.electronics.design that Joerg

It's all a matter of money and they'll do any color you want. Remember
when you could have your car delivered in a color that matched your
wife's hair? I just wonder what happened to those cars in cases of
divorce.

Or the following week, when she changed her colour.

 

[Keith Williams]

Hello Graham,


It's all a matter of money and they'll do any color you want. Remember
when you could have your car delivered in a color that matched your
wife's hair? I just wonder what happened to those cars in cases of divorce.

Nothing changes. She gets the car.

 

[Ge0rge Marutz]

But then you say you need 4 months design cycle for this minor >layout change. Why?

1.) Board layout is extremely dense as it is. I will have to go with
a different micro offered in a smaller outline package and 0402 package
resistors and caps to make room for a few traces and vias that are
currently underneath the processor. The design work, including:
selecting new components, retrieving quotations, circuit analysis,
schematic capture, PCB layout, and software, will take approximately
3-4 weeks.

2.) Need time to get quick turn boards, build initial prototypes by
hand, and bench testing. 2 weeks

3.) EMC test. Must be done at an outside lab. Lab time is extremely
hard to get. I always count on 1 month for this but may get through it
in 2 weeks if we pay VERY high expedition fees.

4.) Design validation testing. A majority of the testing can be
completed in 1 month. However, some will need to linger behind.

5.) Production line change over, test modifications, and production
validation testing. Minimum 1 month.

That is if nothing goes wrong.

Ge0rge

 

[Rich Grise]

I've had micros running faster than that work fine on single-sided pcbs !

I've had stuff running that fast on Vectobord. ;-) Well, admittedly, it
was the type (I forget the number now, 8317 or something) with a ground
plane (more of a grid, actually) on one side and pad-per-hole on the other.

Cheers!
Rich

 

[qrk]

HAH!!! Joerg, you hit the nail on the head. This customer fully
believes that any other color circuit board besides BLUE will not work.
So, I agreed to supply blue circuit boards instead of green.

I offered to make the design change once the small volume product
launch is underway and BEFORE high volumes kick in. Its going to take
4 months to complete the design cycle using the most agressive timing
possible. I have 2 months. He will not buy into this.

I understand he took comments by a trusted collegue. I do not blame
him a bit for this. I just need to find a way to get through to him.
There is more than one way to skin a cat. One method will work just as
good as another in this application. Butter knife vs scalpel.

Ge0rge

You need to present your story to the "trusted colleague" that said
this wouldn't work. Using the Dilbertesque manager as a go-between
will not work. Could you arrange a meeting between the trusted
colleague and you? If you could present your measurements to the
prognosticator, perhaps you could change some minds - or at least
understand where they are coming from.

You could also dazzle them with pretty EM pictures provided by ATLC
<http://atlc.sourceforge.net/> and show that the measly ground you
have on the top layer probably does nothing because you already have a
beautiful ground plane 20 mils away. Also see
<http://www.sigcon.com/Pubs/edn/GroundFill.htm> where Howard Johnson
speaks about the uselessness of ground fills when you already have a
ground plane.

 

[Joerg]

Hello Keith,

Nothing changes. She gets the car.

Ah, right. Didn't Barbie get Ken's car, Ken's boat, Ken's house ...?

Regards, Joerg

 

[Joerg]

Hello George,

1.) Board layout is extremely dense as it is. I will have to go with
a different micro offered in a smaller outline package and 0402 package
resistors and caps to make room for a few traces and vias that are
currently underneath the processor. The design work, including:
selecting new components, retrieving quotations, circuit analysis,
schematic capture, PCB layout, and software, will take approximately
3-4 weeks.

Could you add a couple layers and via through? Or maybe offer them to
place a "custom ground enhancement structure" underneath. Essentially ye
olde copper tape but you don't have to say that.

3.) EMC test. Must be done at an outside lab. Lab time is extremely
hard to get. I always count on 1 month for this but may get through it
in 2 weeks if we pay VERY high expedition fees.

Try labs in more remote locations, preferable smaller ones. It's been a
while but I really liked those, worth the travel. Less bureaucracy,
quick service, friendly and cooperative people. Oh, and during one of
those EMC trips we were able to witness a true to the bones bar fight in
Mariposa (just outside Yosemite). Hoss Cartwright would had gotten a
kick out of it.

That is if nothing goes wrong.

And something usually does...

Regards, Joerg

 

[Rich the Newsgroup Wacko]

Hello Keith,


Ah, right. Didn't Barbie get Ken's car, Ken's boat, Ken's house ...?

And then run off with Malibu Stacie? ;-)

 

[keith]

Hello Keith,


Ah, right. Didn't Barbie get Ken's car, Ken's boat, Ken's house ...?

Sure. "Divorce Barbie"; comes with all Ken's shit.

 

[[email protected]]

One "tough EMC environment" product I was involved with even had a
stamped metal shield "ground plane" over the *top* of the QFP package.

Interesting. Did it have an insulating cover on the inside (towards the
chip and leads), or was it just placed "far enough" away (tall enough)
to not be a problem?

Matt Roberds