# Using a mixed technology, SMD and THT components in one PCB

I want change design from Through-hole technology to SMD design. Some components have no SMD version, for example some transistors in TO-92 package, and Power Mosfets, which available in DPAK SMD package, but since it requires a heat-sink, there is no much benefits of using SMD Mosfets with heat-sinks(DPAK heat-sinks have amost the same size as regular heat-sinks).

For example, I can change all the ICs, resistors and caps to SMD versions, but there still remain Power Mosfets and TO-92 transistors in THT version. Is there any disadvantages of using mixed technology, when use SMD components with some Thru-hole components mixed in one PCB, except more complexity and thus extra cost? In terms of reliability? The quantity is small, 10-20 pcs, not mass production. All components in one side.

• You'll get different answers for different production run sizes. How many boards are you building? 10? 10000? 100000000? – Brian Drummond May 12 '15 at 13:13
• At 10s of units, wave soldering isn't an issue - hand soldering is probably faster than setting up the wave machine, and less stress on the board. Substitute SMD where you can (e.g. most of those TO92s) and simply hand-solder the few remaining PTH parts. – Brian Drummond May 12 '15 at 18:41

Generally your cheapest costs will be if all the parts are SMT and populated on a single side. Even if the parts cost somewhat more, the overall cost and handling may be worth it. If you can eliminate the heat sink by using copper area you may be able to save money and improve quality by eliminating assembly steps (for example, where an assembler could forget to put thermal grease on a part or insufficiently tighten a screw).

In smaller quantities (depending on where they are made too) it often makes sense to populate any through-hole parts manually after all the SMT parts are mounted.

These days often parts are only available in SMT, or the SMT parts are better than equivalent through-hole parts because of superior heat transfer through a thermal pad, say, but more often the SMT equivalents are inferior in mechanical robustness (especially things like connectors and switches), heat dissipation or some other characteristic.

For substituting things such as your TO-92 transistors, for example MMBT4401 is similar to 2N4401 but good for much less power dissipation. I would be very surprised if you cannot find a close equivalent or (more likely) a superior part for the TO-92 transistors.

Edit: You have mentioned a few part numbers in the below comment- 2N3904 and J310 JFET for example. There are no exact equivalents because the power dissipation and package is different, but MMBT3904 is likely the same die as the 2N3904 so it is very similar. MMBFJ310 is very similar to the J310 with small differences (eg. capacitance). In general, a capable engineer should review all substitutions including these ones, as well as any layout changes.

The 27N3LH5 power MOSFET is a more complex design question that would best involve looking at the entire application, but at a minimum would require estimating the power dissipation. In general you should review MOSFET part numbers from time to time as they are still improving and older parts are being obsoleted relatively frequently. Since the part you have has a rather high Rds(on) of 20m$\Omega$ you might be able to substitute a newer SMT part and greatly reduce the power dissipation and reduce the part cost. To do this does require at least the same knowledge and calculations as it took to originally design the circuit.

• For example, J310 JFET Transistor and 2N3904 NPN Transistor is only available in TO-92. Is there an exact equivalents? And analogue for Power MOSFETs 27N3LH5, to use them without heat sinks? – minto May 12 '15 at 15:01
• Thanks for the helpful advices. 27N3LH5 Mosfets are used to control the LEDs brightness, the set of LEDs connected to the output of each channel. Forward current up to 2A per light driver circuit. Power supply +12VDC. The Mosfet connection. – minto May 12 '15 at 17:34
• That's a pretty non-critical circuit. Something like a PSMN9R5-30YLC,115 would be cheap (about 25 cents in 1K) and would easily handle a few A without a heatsink. – Spehro Pefhany May 12 '15 at 17:37
• May also be used existing Mosfet 27N3LH5 in SMD DPAK package without a heat sink? – minto May 12 '15 at 19:23
• @minto Do the math and tell me what you think.. – Spehro Pefhany May 12 '15 at 19:26

Nothing in terms of reliability. There are issues if you're making a board for production, though, in that you may be using two processes instead of one. You may need to reflow the SMD parts, and you may need to wave solder the TH parts. If you have SMD parts on both sides of the board, this may get fairly complicated, and you may need to have certain IC's glued down during placement.

The more processes, the more expensive. If you're planning production, I suggest talking to your assembly guy before finalizing the board. There may be some things he can offer that would make the process easier and cheaper

My company is in the middle of migrating our existing higher-volume PCB designs from completely through-hole to mixed SMT & through-hole components. We currently manufacture several different higher-volume designs with an aggregate total of between 400 - 1000 pcs per month. Averages about 10,000 pcs each year.

Our boards have many larger components that are available in only through-hole versions: large power relays, large terminal blocks, large pluggable connectors (Mate'n'loc family).

Regardless, the move to SMT will result in some manufacturing-cost savings. I do not expect any negative impact on reliability.

The existing PCB layouts are single-sided - the new layouts are double sided with all of the SMT components on the top side of the board. We plan to have our contract manufacturer do all of the SMT manufacturing - we will then do all of the through-hole assembly in our facility using our existing wave solder process.

The advantage of this process flow is that we can have our contract manufacturer do a board run a couple of times per year. The cost to inventory the partially-assembled boards is relatively low because all of the SMT components are relatively low-cost. The major cost in these boards come from the larger through-hole components.

When we start a month's scheduled production run, we will simply pull partially-assembled boards from inventory, stuff all of the through-hole components, wave-solder, clean.

All of our boards undergo a full performance test and some boards require calibration. We then simply apply serial numbers and pack the boards for shipping to our customers.

• Just curious, do you do anything special to maintain solderability on the partially assembled boards that might be stored for weeks or months? – The Photon May 12 '15 at 15:59
• @ThePhoton I don't know what Dwayne does, but we have partially assembled boards sealed in individual antistatic bags by the assembly house. Gold plating- they're just as good after a few months. – Spehro Pefhany May 12 '15 at 20:13
• If we replace LM348 Op-amp with TL074 Op-amp in above circuit, the direct replacement can be done without changing the passive components? – minto May 21 '15 at 13:00