[SOLVED] Through-hole to SMD conversion (pcb)

[sharikbaig]

Hi, Hope you all are doing good,

Say I have a PCB, a through hole one and i want to convert the same into SMT using as much SMD's as possible. Now the question is, are series & parallel passive component values same for SMD's? For example, if i used a 330 Ohm resistor in series with a LED to attain a specific brightness at 5V, in SMT will that resistor be of 330 Ohms or will these values change? Are current ratings of all SMD's are same as Through hole devices? And as we know 78XX series usually require heat sink, sure there isn't any provision to attach heat sink to a 78XX in SMD form, so how do we tackle the heating, or does it not heat in SMD package?
Apart from these, are there any more similar complications?

Thanks in advance

 

[betwixt]

Re: Through-hole to SMD conversion

All the component calculations, at least up to very high frequencies are identical. When the frequency is very high, the calculation stays the same but the effects of wire leg inductance and package capacitance may have to be taken into consideration.

Heat sinking with SMD devices is normally achieved by soldering the tab to a copper area large enough to disperse the heat. Sometimes it is beneficial to use copper on both sides of the board and use many via holes to aid conduction through the substrate. In extreme cases, an additional heat sink may have to be mounted on the PCB to help dissipate the heat. The same heat is generated regardeless of the package type though.

Brian.

 

[KlausST]

Re: Through-hole to SMD conversion

Hi,

you talk about LED.
for same brightness you need to consult the datasheets of the LEDs.
Either you need to choos LEDs with the same brightness at same current, or you have to adjust current (resistor value) to a new brightness.

Most parts can be switchend from THM to SMD, but in detail you need to consult the datasheets.
It really depends on the parameters you want to focus. (voltage rating, size, capacitor dielectric material, tolerance, drifts, power dissipation and thermal resistance, noise, ...)

Klaus

 

[asdf44]

Re: Through-hole to SMD conversion

For small amounts of heatsinking SMD can be as good or better compared to, say, a TO-220 in still air because one way or another the board will spread some heat (as said, copper pads and vias can be used).

For a couple more watts of heatsinking there are surface mount sinks for SMD components, particularly the various DPAKs (the SMT versions of TO-220 style components). This may be good for many 78XX regulator applications.

See here:
https://www.digikey.com/en/product-highlight/a/aavid-thermalloy/surface-mount-heatsinks


For perhaps 5-10 watts or more through hole components with bolt or clip-on heatsinks are probably still the way to go (though some specialized SMD components are designed to have heatsinks pressed to them from the top).

 

[sharikbaig]

Thanks for your replies Brian, Klaus & asdf. Your answers were really helpful. Looking at the data sheets, almost all of the ratings of SMD match with THM. Brian & asdf thanks for sharing your knowledge on heat sinks.

 

[mshh]

Is there difference in dissipated heat from smd or through hole mosfets? As i want to solder a through hole mosfet to tge pcb

 

[KlausST]

Hi,

The dissipated heat (power) won´t change....but the temperature rise will change, because of different R-th.

Klaus

 

[mshh]

Could i use irf3205 without heat sink if the current is 1.5 ampere at frequency 20khz?i mean i will solder its back to the pcb

 

[betwixt]

That depends on many other factors such as the on/off time ratio, rise and fall times of the drive waveform, drive voltage levels and whether there is any reactive part in the load.

Brian.

 

[mshh]

it is on/off every 20 seconds and it lasts for 5 sec. the drive voltage level is 12v. the load is dc motor

- - - Updated - - -

it is written in this irf3205 datasheet that "The D2Pak is suitable for high current applications
because of its low internal connection resistance and can dissipate up
to 2.0W in a typical surface mount application.
The through-hole version (IRF3205L) is available for low-profile
applications."
How could the smd be suitable for high current applications? However they have the same datasheet.