Choosing SMD size for passive components

Question

The passive components like resistors, capacitors, inductors are made up of a multitude of materials and come in a very wide range of package sizes. We could get the same value of resistor, capacitor and inductor in various sizes. I assume that smaller means that we pay more for the same value component e.g 10uF capacitor in smaller packet might cost more than a larger package.

Now my question is, how do the design engineers decide on what package to use in the design? We have things that can go down to 0201 and even smaller e.g 01005. If the design requirements say make the smallest PCB possible, then we would like to use the smallest components. But how is the decision made otherwise? I am sure there is a simple answer to this.

Answer 1

General Principles

It's generally a "sensible and reasonable" kind of judgement question, not an absolute one.

The considerations include:

• Are the parts large enough for their power/voltage requirements?
• Which are the cheapest?
• Sometimes the larger ones are more reliable
• They will have different thermal properties
• If you're making it by hand, you don't want them too small
• If you anticipate that you might need modifications, bigger is a lot easier
• Your assembly company will have limits
• Your assembly company might give you the most common passives for free (because they're already on the machine)
• How small are your voltmeter/oscilloscope probes?
• Are the parts large enough for component values to be printed on them?
• If the board suffers vibration or handling, it may be more robust if the parts are larger
• Durage a parts shortage, some sizes may be more likely to remain available
• Capacitors and inductors have a lot of other performance variables beyond capacitance and inductance (ESR and current rating being just two) and appropriate ones may only be available in certain sizes

In practice, designers typically have a palette of parts and sizes they have good experience with, and they tend to stick to those. Even if a production circuit might have a free choice of components, development and prototype boards might want to use components the designer has in stock.

Some PCB assembly companies offer ranges of standard parts for free, on the grounds that the parts themselves are cheaper than the time to put in reels of components. Checking a large UK assembly company, the resistors and capacitors were in the range 0201, 0402, 0603, 0805, 1206; ferrites in 0805, a few transistors and diodes in SOT-23.

Survey

I took a survey at a major US distributor for its selection of 10 K SMD resistors with 1% tolerance, to see what sizes it expects to sell.

It had a total of approx 80 million parts in stock and about 400 product lines, with 97% in the range 50 mW to 250 mW; the most common were 100 mW (30m parts, 80 varieties) The highest level of stock was for the size 0603 (all sizes here are inches), with about 30% of 10K 1% SMD in this size, and ~90 different product lines. The highest stocked individual part numbers were 0603 (100 mW) with 12 million in stock, 0805 (125 mW, 7.7m), 0402 (62.5 mW, 6.4m). 0201 and 1206 were about 3m each, at 50 and 250 mW respectively. 0201 was 200 ppm/deg C, all the other 100. All of these were the same price, about USD 4 per 1000.

Thus, after checking whether you have any special requirements for power or anything else, most designers appear to plump for 0603. My inference (from stock levels vs number of product lines) is that we're seeing a shift away from 0805 and an increase in 0402. The high level of choice and low stock levels for the larger sizes (1210 and up) suggests they have niche and or obsolete purposes.

Answer 2

Nope, there's not a simple answer to your question. Like many things, "it depends."

Smaller packages do not necessarily incur higher costs. Cost is linked to supply and demand, complexity to manufacture, and a myriad of other factors. As just one example, with ceramic surface mount capacitors, more capacitance or higher voltage tolerance will add cost for a given package size. But a "bog standard" 0.1 µF capacitor with a common or low voltage rating like 6.3 or 10 V can be found at essentially the same price for a variety of package sizes.

A more important factor in choosing package size is your fabrication capabilities. Are you hand-assembling a few boards at a hobby level? Avoid the ultra-small packages! Are you mass-producing a commercial product where PCB real estate is precious? The smallest packages would be essential in such a case.

Component size is a factor in both desirable properties as well as parasitic or undesirable properties. Larger capacitance values translate to larger physical sizes. Higher current ratings in inductors also translate to larger physical sizes. For example, if you're building a switching power supply, you must choose an inductor that is rated for the intended current, which leaves some smaller package options out.

In the case that all of this is irrelevant, and you can choose any size you want for some component, you may want to consider rework and testing. If your fabricator has no problem assembling the smallest 01005 components, do you have any requirements for testing and repair work on the product? Is your fab charging more for high-density interconnect (HDI) PCB assemblies? Unless you need your product to fit some physical constraint, consider assembly and rework costs in addition to component cost.

Answer 3

It depends how much you want to do effort to put weight on some feature.

If you want the smallest possible PCB, you must use smallest possible components, and then the fab that mounts the components on will have to use a machine capable of handling smallest possible components with required precision, and also the PCB must be manufactured in a fab house that can make the required track size and drill hole sizes with enough precision to do it. It may also take longer to find a slot to use the most precise PCB manufacturing line or most capable pick-and-place machine.

So if you want the PCB cheaper and faster, you ask your PCB fab house and the EMS who mounts the components what they can do, what are cheap normal size components they have in stock and what size is a costly special order.

If you want the smallest size and least weight PCB with smallest components, be prepared to pay more because you want something that is uncommon.

Answer 4

Most companies have a standard size they default to when there are no requirements dictating smaller or bigger sizes. Having a default size makes many things a lot easier, e. g. maintaining a small in-house-stock for repair and prototyping.

The default size depends on the company's typical applications and other aspects like availability, of course. In most companies the default size is probably 0603 or 0402 nowadays. One obvious reason for 0603 and 0402 is that these should be "pain-free" for mass production for any half-way decent assembly house and still small enough for medium density boards.