Tips/Gotcha's when converting a design from through hole to surface mount

[nakchak]

I have never been very sucessful finding any good resources about converting a design from through hole to smt.  Its something i have personally never done as its always seems like a subject which would be filled with gotcha's and painfully hard earned knowledge.

Would love to see a show on that subject...

[free_electron]

the key points are going to be the power dissipation in parts and the standoff voltages.
smd resistors are smaller so they donlt have the power rating of their through hole counterparts.
the same goes for standoff voltages. a 1/4 watt resistor may hold 250 volts.. an 0805 can't . it'll flash over in the trimming cut.

If the product is for production on automated assembly line then other factors come into play. parts top , or top and bottom, two reflow steps , single reflow step.

For smd connectors you need to look at mechanic strength , especially for user accessible connectors.

[vxp036000]

One important consideration, if your design is above a few hundred kHz, is that your board was tweaked to accommodate parisitics.  When the design is implemented on a surface mount board, the parisitics will be entirely different and the design unlikely to perform as expected.  So, for a board with any RF frequencies, some amount of re-design could be required.

[T4P]

When it comes to connectors like USB which has not 1 type but oh so many types of connectors , the Mini and Micro variants can be SMD and would last but do the same for USB and the connector will break off the joint in a short while .

[SeanB]

I would add that going SMD will mean a dual layer board at least, as there are no longer any spaces between component leads for routing traces. You have to be very aware of capacitors and the variation of capacitance with voltage as well as type with ceramic devices ( most common SMD capacitors) and the tempco of them as well. Your board may or may not be much smaller, as there will be much more traces and vias than before. You will most likely still be mixing leaded and leadless components though, very few boards are totally through hole device free.

[Neilm]

Let me start with an obvious statement - SMD components are small, so the pins are closer together. This means that you can get leakage issues - especially if you have any high gain amplifier circuits or have not connected trim pins.

Another thing to be wary of is that some components might have the same part number on the outside on both through hole and SMD parts, but have very different characteristics in some parameters. Usually on the 10th page, buried in the text but it will completely change the parts performance in the application you are using. One example of this is a linear supply I used many years ago - the through hole part could output 0.5 amps, the SMD one only 200mA.

Neil