What is the significance and use of different heights of substrate and copper claddin

[hari_preetha]

What is the use and significance of different heights of substrates (Such height as 0.508mm,0.256mm,0.625mm,0.762mm,1.026mm,1.270mm,2.540mm) and also different thickness copper cladding around the substrate (Copper cladding thickness around the substrate such as 0.017mm,0.035mm,0.070mm).
Why do we have different heights of the substrate and different thickness copper cladding around substrate. Is there any use or disadvantages to have different heights of substrate and different thickness copper cladding around the substrate.

 

[thylacine1975]

Electrically, varying thickness substrates allow different circuit impedances to be realised. Mechanically, they permit a specific finished board thickness/strength to be achieved.

Increasingly thick copper cladding permits higher circuit currents without excessive track heating - at the expense of etching/manufacturing tolerances. Occasionally a semiconductor manufacturer will specify a minimum copper thickness for adequate heatsinking of components in tight spots.

 

[D.A.(Tony)Stewart]

Re: What is the significance and use of different heights of Cu clad PCB

The main upside to have 2oz copper is to reduce ohmic losses in high current tracks and use 1/2 oz for inner signal layers in high stack boards to keep them thin. The outside is normally plated with an extra 1/2oz copper . This 4:1 range in copper thickness options is universal, but others may have more options.

The odd looking metric thickness or height in a stack for each layer, is from metric conversion for Oz per area of Cu.

The main downside is the bulk cost in any high volume PCB is the total copper weight of all layers, unless a design adds many costly special features beyond normal designs. Low volume has setup costs that outweighs copper weight costs but still adds cost.

For example making a 6 layer 2 Oz (ounce per square') board would be approximately 5 times the cost of a 1:½:½:½:½:1 copper stack using 1/2 copper throughout and electroplating the outer layers for reliability.

Heat transfer can also be improved with Alum. Clad boards with epoxy coating and copper plated tracks with normal solder mask on top.

The other tradeoff is called etchback factor which determines minimum trace size and tolerance which rises with thickness due to etching of edges.