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Old 13th August 2006, 01:17 PM   #1
Clipped is offline Clipped  Thailand
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Default pcb thickness compensation

hi, if i use a design that was first built using a 4 oz board, and then change to using a 8 oz board, would i have to compensate for the extra thickness in any of the circuits?

and would different pcb materials really make a difference in sound quality?

i was also thinking about getting the pcb gold plated, for looks...but would there be anything i would have to watch out for if i did this?

if so, what do you guys think the best material would be? this is going to be for a high current car amplifier. around 500 watts.


thanx
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Old 13th August 2006, 01:57 PM   #2
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Thicker copper just means more current capacity and harder to solder. Gold plating also makes it harder to solder.
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Old 13th August 2006, 04:46 PM   #3
poobah is offline poobah  United States
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Each ounce of copper tranlates to 0.0014" per side. So adding 4 oz to each side of a board would make the board about 0.012" thicker.

This assumes that your board maker does not change the thickness of the base material to compensate for the copper build. Your board maker likely has many options as to what material they start with though.

The general rule for safety's sake is to allow 0.040" in width and 1 ounce in thickness per amp (rms) of current flow.

I do boards with up to 12 oz plating... there are lots of problems. 4 oz is about as thick as you can go before you start running into geometry problems with typical surface mount components.

As the build gets thicker; the minimum gap between traces, and the minimum width of traces both must increase. Also, the size of plated-thru holes becomes more difficult to control... so you opt for greater hole size to combat this. Warpage becomes a problem as build increases due to uneven copper build; you may have to use a method called "thieving" to avoid those problems.

Your board shop can explain all the limitations as they are usually a function of their particular process (there are lots of them). It is possible to have different thicknesses of copper on different areas of the board. This requires a separate masking and plating step. Tricky, but doable, won't make you any friends at the PCB shop, except for the owner.

As far as material choice is concerned, unless you can hear into the gigaHertz... forget about it... G-10/FR-4 is fine.

Circuits are plated in gold for 3 reasons. First, because gold is flatter then the alternatives; this helps when large SMT components with fine geometry are used. Second, because gold does not oxidize, it allows for very mild fluxes during the soldering process, or it permits poor handling and storage of the boards where alternatives would have corroded. Third, it is used for edge contacts (where the edge of a board pokes into a motherboard) gold makes good contacts, although not durable, again, becuase it doesn't corrode. In either event, the copper has to be "flashed" with nickel before the application of gold, otherwise the gold and copper diffuse into each other over time.

Keep your thickness 4 oz or less... forget about the gold... you'll be fine.
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Old 13th August 2006, 04:48 PM   #4
jcx is offline jcx  United States
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heavier copper also reduces etching resolution - you will have larger minimum space/trace numbers - not likely a problem with 50 mil lead space soic, but maybe for tssop

also only a few boardhouses will actually take your order for 8 oz copper
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Old 14th August 2006, 10:22 AM   #5
Clipped is offline Clipped  Thailand
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thanks, all points noted, instead of going with 8oz.

i think i'll just apply 22 guage solid strand wire soldered to the important traces...

looks like there are going to be alot more problems than this novice can handle on his first attempt...

my main concern is heat build up on the board, more than likely i'll go with FR4...but if there is a slightly better alternative which you could name, i'll go with that...keep in mind that there is going to be switching power supply on board.

oh and one more thing, when i get them to cut/route the boards apart from the master blank...would it be possible to have two boards connected together?

example, say a blank has space for 4 boards, instead of cutting it into 4 pieces, could i get it cut into 2 pieces? and have 2 amps on 1 board?

im scared theres going to be some interference somewhere....would it be best to have the smps at different ends or would it be ok to have them both oriented towards the middle of the pcb, with the outputs on both ends? this is a car amplifier with the fets/transistors located on the sides.



thanx!!!
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