# How to find SMD resistor and capacitor value and correct size

I'm new to the SMD world and struggling to find an easy and quick way to get the correct SMD resistor and SMD capacitor size and value based on a circuit schematic; any tips will help.

For example, I'm designing this circuit in SMD but don't know how to find the right SMD size, because there are many sizes - (I'm using Altium Designer for PCB design.)

If someone can guide me by looking at the resistors' and capacitors' values from the above diagram and tips on finding the correct size, it would help me further.

Smaller sizes tend to work better at higher frequencies and are easier to fit into a good layout. Larger values tend to have high power ratings and (in the case of capacitors) have more stable capacitance under bias voltages. Larger are easier to solder (if you are doing this by hand) but take up more room.

If you are a beginner and soldering this by hand, 1206 or 0805 are good sizes to use. Smaller is not too much more difficult to work with, but no sense making things harder for yourself unless there is a good reason. If you are paying for assembly, and they can do smaller sizes, this may not matter.

When picking resistors, pay close attention to power ratings. Don't exceed the maximum dissipation. If you do, pick a bigger size.

When picking capacitors, pay close attention to the change in capacitance with voltage and the impedance at high frequency. For example, here is a random 1 uF 0402 capacitor:

And here is the same 1 uF but in 1206:

(images from: https://ds.murata.co.jp/simsurfing/mlcc.html?lcid=en-us)

Comparing capacitance at 5v, the larger package works much better, but it also has higher impedance above ~ 1 MHz. At higher frequencies, a larger package size may not be appropriate.

• Great points, especially the hand-soldering bit. Though it probably is worth mentioning OP's circuit isn't gonna operate anywhere near the frequencies that those graphs will matter. Don't make him/her panic ;) Jan 28, 2021 at 6:07
• You and @Kyle B both gave me the answer and both answers are understandable. I think I understand now that. Maybe I can create a calculator for other users to make life easy? I appreciate your time :) Jan 28, 2021 at 7:03

Some reasons to use specific sizes....

1. What size resistors can your manufacturer work with? Not every board house can place an 0201 resistor.

2. There has been a shortage of some materials which are used to make SMT resistors & caps recently, resulting in a large cost increase for bigger components. In our company, we changed from 0805 to 0603 or 0402 everywhere.

3. Voltage holdoff... smaller components can not be subjected to as high a voltage as larger ones. They may arc-over or fail internally (Rarely an issue with logic-level stuff like you show above)

4. The MAIN issue is POWER DISSIPATION. You need to ensure your resistor can handle whatever power it is required to dissipate. There is a chart on this page about half-way down

https://www.electronics-notes.com/articles/electronic_components/resistors/surface-mount-smt-smd-resistor.php

Capacitors don't really have as much issue with #4 above, but #1, #2 and #3 still definitely apply. You really gotta be operating at high voltages or frequencies to heat up a cap like the ones you're referencing.

• You and @user1850479 both gave me the answer and both answers are understandable. I think I understand now that. Maybe I can create a calculator for other users to make life easy? I appreciate your time :) Jan 28, 2021 at 7:03