choosing surface mount resistor/capacitors

Question

So I need design a prototype PCB circuit and need to decide on using what kind of surface mount resistors and capacitors to use. But looking at digikey, there are so many types with many variations in size, manufacturers, etc.!

Since there is not really any strict requirements for my project, I'm assuming most of them would work fine. Cost and precision is not a huge issue for my case.

But what are some deciding factor on choosing which size/packaging/manufacturers to use?

Answer 1

For assembling by hand I offer the following suggestions:

• Capacitors: For the ubiquitous 0.1uF ceramic decoupling caps go with 0603. They're nonpolar and unlabelled so the small size is not that much of an issue. You could buy a whole reel and be set for life. Every tantalum and electrolytic SMT cap is big enough that handling is not an issue. If you are following a datasheet recommendation to put down 2 or 3 decades of caps near a pin of a high-density package even 0805 will feel very big.
• Resistors: They are labelled down to 0603 (usually). If you care about reading the markings it's worth checking a few with your own eyes to see what you can read. I find 0805 much more readable than 0603. A nice tradeoff for density vs handling is 4 resistor arrays. A 1206 array is basically 4 0603 resistors side-by-side. The package is much bigger and the label is much easier to read but the density is even better than 0603 (because there is no gap) and the soldering is only slightly trickier. These are great for pullups, series/stub terminations, LED arrays, etc.
• Jumping over tracks: It's easy to home-etch a board with a trace going under (between the pads) of a 1206 or 805. You can manage nearly 10/10 rules cutting under 0603 but you will probably have to make your own land pattern for 0603 to do it (otherwise the pads will be too close).
• LEDs: I find the ones in the tantalum-capacitor style A, B, C, D cases the easiest to orient and solder. Ones sized like resistors (0603) can be quite hard to orient without magnification. I usually use a setup where I can probe the LED to light it up to verify which way it's going in.
• Other ICs: SOT-223 and SO-89 are nice for voltage regulators. SOT-23 is fairly easy to work with in 3-lead packages and slightly trickier in 5 or 6 lead. SO-8 is easy to work with. QFP with pitch 0.8 or 1.0mm is pretty easy but fine-pitch QFP (0.65, 0.5mm) requires a lot of flux and good technique. QFN, MLF, BGA and the like are very tricky to work with at home and can be difficult to route on 2-layer boards (especially without plated through-holes). I avoid PLCC when I can because it's neither small nor that easy to solder (it's very easy to bridge pins under the chip and very hard to desolder them).

Mind your ground relief when connecting to surface mount pads. If you let your plane simply merge with a pad it can be very hard to solder even with a very good iron.

Remember that putting SMT components in your PCB design is almost free. There's no hole to drill and it's just a modification of the copper and silk masks. It won't affect the cost of the PCB at all. If you think you might need another cap or an indicator LED or a pullup resistor just put them all in the design. You can always DNI them.

Answer 2

In the past I've often used parts from Yageo and AVX. There was not really a preference, most of the time the choice was determined by lead time.
As for size, last time I ordered parts 0603 were the cheapest, but you say that price is not an issue. If space is a premium it's obvious that you'd rather choose 0402 parts, that is if power consumption is not a problem. If you have plenty of room you may want to use the older 0805 parts as they're easier to work with, measuring-wise and rework-wise.
1% parts are only slightly more expensive than their 5% counterparts, but they have the advantage of better reproducibility; individual devices (assemblies, not only components) are more likely to behave exactly the same.

Answer 3

You do have to do some due diligence before choosing parts, namely:

• are the parts rated for the expected power dissipation?
• are the parts rated for the expected voltage?
• is the design manufacturable?

Some generalizations:

• 1206 resistors: 0.25W maximum dissipation, 200V maximum
• 0805 resistors: 0.125W maximum dissipation, 150V maximum
• 0603 resistors: 100mW maximum dissipation, 75V maximum
• 0402 resistors: 62.5mW maximum dissipation, 50V maximum
• 1% resistor tolerance is good enough for most applications, select 0.1% Nichrome resistors if accuracy is needed
• Go with X7R or X5R dielectric material for the surface-mount caps, for a good compromise between thermal drift and cost. NP0 or C0G is good where less drift is desired
• Many surface-mount MLCCs are good for 50V (down to 0402)

Answer 4

The main consideration is whether you are assembling the board yourself or not. 0805 Rs and Cs are best in the former case, being relatively easy to hand-solder and not taking up much room. If the latter, the assembly company will usually supply the passive components. It would be a good idea to check what they are using but a reputable company selects appropriate parts, in my experience.

Another important thing to consider is RoHS vs non-RoHS.

Answer 5

Here is a summary table describing various types of capacitors. This is from The Art of Electronics by Horowitz & Hill. Reprinted without permission. This may help you to choose the best type of capacitor for your application.

Another factor to consider is capacitor frequency "response". I should be clear that this is not referring to frequency response in the usual sense (i.e. $X_C=\frac 1 {j\omega C}$). Different dielectrics are generally only usable over a certain frequency range. This page has a chart and a short description. I believe this is why two different types of caps are often paralleled even though one may have a much smaller capacitance than the other, such as a 0.1uF ceramic cap in parallel with a 1uF electrolytic or tantalum.