Replacing through-hole type components with SMD components 1

[sanvik]

I want convert existing PCB design from through-hole type design to SMD design.
Is clear about selecting IC in SMD package selecting, but not fully clear about passive components: trimmer potentiometers,
Electrolytic Capacitors and resistors. There are 10 trim-pots are used to adjust frequency: 5pcs 50K trim-pots and 5pcs 5K trim-pots, and one 50K POT is used for adjust input signal level. Normally, it requires only once to adjust this. What type of SMD trim-pots can be used to replace these 50K and 5K through-hole type trim-pots? The SMD trim-pots are very small, not sure how handy is to adjust it. Also, what type of SMD capacitors and Electrolytic Capacitors can be used to replace existing ceramic and Electrolytic Capacitors?

 

[ScottyUK]

SMD equivalents exist for pretty much everything on that board except the screw terminal connectors and perhaps the fuse carrier. SMD ceramic caps are commonplace, and small electrolytics are available in SMD designs. Have you looked at any of the major component suppliers?

 

[sanvik]

Yes, I looked for some SMD versions of that components. In particular, I am not sure about SMD trimmer potentiometers: although they are not intended to use it very often, they are extremelly small size 5x5mm, and seems it will be very difficult to tune/adjust it with screwdriver. What's power rating of a resistors can be used? Power supply is +12V, each channel support up to 2A per light driver circuit.
Also, not sure about using SMD Power MOSFET 27N3LH5: there are used TO-220(DIP) package, and SMD version is DPAK: the difference is that DPAK have Total dissipation(P tot) 30Wt whereas TO-220 have 45W.

 

[itsmoked]

A better question might be: Why exactly do you want to change the technology?

There are several points you should consider:

How many do you build each order cycle?
When you have SMD boards produced you have to build at least 100 or the assembler will screw you mightily.
With thru hole there is no penalty for having just a few assembled.

Parts usually cost a little more in SMD form. Often resistors cost less but everything else more. Those
aluminum caps will cost probably 2X more.

Having ANY thru hole parts on an SMD board requires the assembler to run the board thru both of the
technology assembly paths. This ALWAYS adds considerably to the cost of assembly.

Having to heat-sink parts on an SMD board adds a lot of complexity and over a certain power dissipation is
not applicable at all. This is why those large, leaded TO-220 parts are still popular.(more thru holes retained)

Lastly, if you are going to the expense of all new parts and laying out a new board you should dispense with the
archaic pots due to their cost, the labor they require, and the early failures they represent by updating the design.



Keith Cress
kcress -

http://www.flaminsystems.com

 

[sanvik]

Its for myself, not for mass production, though. SMD PCB is much smaller and looks better. I can stay with through-hole design though, just need make new PCB a little smaller.
Design it not mine. Which terminal block model is usually used in such design for wire-to-board wiring? (commom Pitch, mm).

 

[itsmoked]

No standard on spacing.

Put those resistors on end and save at least an inch.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[Skogsgurra]

I like Smoked's pragmatic approach. If a smaller board is all you need, then do what Keith says.

You can also rise the power transistors with their heat sinks and perhaps use a quad opamp instead of four separate ones. There's also a lot of naked board area that can be reduced.

One off, you say? SMD? You work at Fort Knox?

Gunnar Englund

http://www.gke.org

--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

 

[sanvik]

No standard on spacing.
Put those resistors on end and save at least an inch.

Which resistors do you mean?
Its Terminal Blocks, there is pin spacing (pitch).

 

[IRstuff]

He probably means ALL the resistors; if they were all laid out in a single row, you'd get a considerable amount of board size reduction

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[itsmoked]

All the resistors! It's a standard practice to save board space by putting all thru-hole resistors on end.

Yes, on terminal blocks there is no standard pitch. There are:
0.098
0.100
0.138
0.146
0.150
0.156
0.157
0.177
0.180
0.197
0.200
0.250
0.276"
and the list goes on..

Do keep in mind that as the pitch gets smaller they can get harder
to use and may need special small screwdrivers. They also get weaker so
wire strains can damage them if the wires gets yanked.

Knowing now that you're only doing one board which means you'll probably
be assembling it yourself I retract most of my concerns about mixed SMD
and thru hole since production aspects are null-and-void.

1) Go ahead and use SMD resistors everywhere. (Make sure you consider the power
dissipation of the smaller SMD resistors)

2) Use SMD caps for all the little decoupling caps.

3) Use SMD LEDs.

4) Change all the ICs to SMD versions.

5) If wire size and loading will allow maybe drop the terminal block size one major pitch.

6) Consider replacing the electrolytic caps with surface mount ceramics - NOT surface mount
electrolytics. If the ceramics won't make the reach do NOT switch to SMD electrolytics,
stick with the thru-hole ones.

7) The TO92 transistor can be switched to SMD if you can deal with the power dissipation question.

8) Keep the same pots and keep the big power parts and their heat sinks, also keep that fuse style.

Have fun.


Keith Cress
kcress -

http://www.flaminsystems.com

 

[MacGyverS2000]

Every one of those ICs likely has a SMD version that is 1/4th the board space as its TH parent. Very little heatsinking going on with those, and SMD versions are often served just as well with a ground pad and lots of capper on an inner/outer layer. I could make that board 1/4th its current size and STILL keep your pots... though I would try my best to design them out. If it's a one-off, measuring for the proper resistor value and inserting that instead of a pot is better anyway.

Dan - Owner

http://www.Hi-TecDesigns.com

 

[zappedagain]

Personally I hate the pots with sunk adjustment screws. The adjustment tool slips off too easily. I've had very good luck with SMD pots with 'normal' adjustment screws, like Copal ST5 series. We use a Vishay adjustment tool and they work well.

Watch out for 'hidden' thru hole components; although probably not a problem as this is your design. In 1994 I inherited a radio design that had just changed from thru-hole to SMD. it had poor reception according to the spec and a capacitive load in the IF section that made no sense. It really needed an inductor for the proper impedance in the IF circuit, but we were told that 'inductors were too expensive' and 'the old design didn't need one'. I helped another engineer tweak the circuit for weeks to get marginal performance. About a year later while cleaning out an old lab I found a sample of the old radio. The thru hole capacitor had one leg wound into an air core inductor and the engineer who converted it to surface mount never caught it. Grrrr... (C) 2015 because I should write this up for EDN one day.

Z

 

[sanvik]

@itsmoked Thanks for the helpful advices! Will try to follow it.

Those TO-92 transistors - 1 x FET J310 and 5 x 2N3904 is only available in TO-92 package.

Power MOSFET 27N3LH5 have DPAK SMD package, but with less Total power dissipation 30W, these Power MOSFETs drives the current (1W LED diodes connected to the output).
There is a Surface mount heat sinks available for D-PAK (TO-252) package semiconductors, but they are wider (0.75") than the existing heat sinks.

@MacGyverS2000 Yes, exactly I'm thinking to find a proper resistor value and replace trim-pots with single resistors. Trim-pots used to adjust center frequency.

 

[sanvik]

Is there anything else wrong in mixed technology, when use SMD + some Thru-hole components in one PCB, except more complexity and thus extra cost?
In terms of reliability?

 

[IRstuff]

Depends on the environment. How much vibration and temperature are you expecting

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[sanvik]

Normal conditions, no anything special.

 

[BrianG]

Hi Sanvik,
You said in an earlier post that you are 'making this for myself', well have you considered how you will assemble it?. Unless you are having a one-off p.c.b. made and assembled professionally (with all the usual methods such as glue spots, solder paste, hot air reflow etc.), have you actually tried hand-soldering surface mount components? Some of the ICs, and especially the single transistor packages are almost impossible to handle, and even with the smallest soldering iron can be a bit problematic to solder first time without getting solder bridges on such close lead spacings.

 

[itsmoked]

I suspect there won't be any issues with switching or mixing SMD into your single board.

Do keep BrianG's points in mind though all the parts you contemplate are on the easier end of
SMD soldering.

Both the FET and the 3904's have SMD equivalents.

Depending on the circuit design and accuracy needed, adjustment via 'resistor selection' could easily NOT work.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[IRstuff]

"Normal"

Seriously? Normal for me is MIL-STD-810. What is "normal" for you?

TTFN
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[zappedagain]

For low volume SMD and thru-hole can go on the same board with ease. At higher volumes they each need a different soldering process (hot air reflow for SMD and wave solder for thru-hole) so that drives up the time to build and thus the assembly cost.

Z