How to factor in board real estate occupied into component prices?

[David_]

Hello.

I am slogging through digikey.com and mouser.com through hundreds and hundreds of components to find those that do fit my needs, but as I was thinking about the price of a 0805 size ceramic 10µF 25V capacitor and one equal to that only in a 0603(cost more) package instead of 0805(cost less) I begun to wonder this:

Does the fact that 0603 means in some way that it COULD be cheaper than an 0805 even if the buying price for the component itself if higher?

Relating to 0805/0603 smd packages the difference might not be so great but I am asking this question in a general manner so don't see this capacitor example as having any importance as opposed to other components.

Say that I had a price list for producing a PCB, how would I factor in the cost for the space that each component will occupies? and is this something that is done in the industry? I mean is there any benefit in doing so?

I'm not sure that it is valid to calculate the cost of each 0805 sized piece of PCB board to then add to each and every 0805 component on the board...

Regards

 

[David_]

I may have formulated my question weird, so let me just ask you this:

Do you take component area coverage into consideration?

 

[FvM]

Your 10µF / 25V capacitor example refers to a case where the smaller form factor already pushes the limits of present technology. The 0603 size is made only by some manufacturers and can achieve a higher price. Most designers will have rather 1206 or 1210 than 0805 available in their company part libraries.

For most passive components, 0603 (or 0402 in some design sections) is a standard form factor, with same or even smaller price than 0805.

Reasonability of using the "extreme" small MLCC is rarely a matter of PCB costs, but may be driven by application size constraints.

For a meaningful discussion, we need to talk about application range, specific size and other requirements.

 

[David_]

Okey I see, yes I did notice that the 0603 10µF was very few in comparison to others.

What has gotten me to want to go this way is decoupling capacitors for digital and mixed signal ICs such a DDS chip, 16- & 18-bit data converters with accompanying voltage reference. And for my current primary project I am trying to design a LCR meter in two versions, one relying on an ADC sampling timed with the excitation signal generator(DDS, AD9954) that should be working through frequency's of 50Hz - 1MHz. and the second version is using RF detectors and a less stringent ADC scheme measuring the detectors DC output and this version will do LCR measurements over the entire DDS chips output frequency range which I think is up to 160MHz.

In this design I have been thinking that I should go all out on decoupling and follow the advice often given in datasheets but that I at least in my hobby adventures haven't seen used in reality even once, datasheets often recommend using 2 or even 3 decoupling capacitors and I had thought to use 10uF, 0,1uF and 1000pF capacitors as the standard decoupling scheme and as such it appeared very attractive to go for 0603 packages but I need to decide how important that is since 0805 is cheaper.

I haven't bought and used very many ceramic caps but I have begun creating an assortment of components to be used for all my project(as far as the fit the requirements) so I have been buying caps 100-500 at a time and first I went for 1206 packages and those turned out I felt to be very large, larger than I really was happy with.
If taking in account the small ICs of today(16-bit DACs in SOT23-8 is rather small) and op-amps in sot23-x and SC-70 packages then 1206 starts to look huge and I feel they increase the over all occupied area of parts that could fit much more snugly if 0603's was used.
As well I have bought resistors 1% in 1206 but I did feel with them as well that 0805 would have been a more suitable choice, but then again don't think I am really in deep enough to know the real results and implications but as I have seen it then if we do not consider the cost and only packages, isn't smaller always better?
If the power needed to be handled by the components isn't excessive, smaller caps would mean smaller parasitics no?

Regards

 

[FvM]

Thinking about ESL series resonances and effective frequency range of the different decoupling capacitors suggests that you don't necessarily need 10 µF immediately at the IC supply pins. You may consider to place values like 1 or 2.2 µF in the proximity and 10 µF (if needed at all) elsewhere.