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Double side board with SMDs. Is it possible to use an oven?
Posted by Plasmateur on 20 Apr, 2021 23:30 -
Hello, I have some double sided boards that I need to populate with SMDs but I don't want to use a solder iron. (in attached pictures)
My soldering skills are only semi-decent and would like to move on to other methods which I've seen online where people place their SMDs and then simply put it in an over. The problem is, I have a board that needs to be populated on both sides, and I'm kinda scratching my head on how to approach this using an over. Alternatively do people use any kind of high intensity light to transfer heat to a surface? This method seems like it might work, but want to ask people who are more experienced than I on how they would approach this.
Thanks!
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I do double-sided boards all the time with a toaster oven. BUT, I have a thermocouple to sense board temperature, and a ramp and soak controller that can make the board follow the temperature profile for reflowing the solder.
You apply solder paste to the least complicated side, place parts and reflow, then repeat for the other side.
I always use a solder stencil to apply the paste. Doing it with a syringe will be quite tedious. Then, applying the parts requires some kind of stand/rest to keep your sleeve from knocking the parts out of position. (I have a pick and place machine for that.)
You need some kind of support to keep the first-soldered side of the board from touching anything during the second reflow. The surface tension of the solder will keep the parts from falling off the bottom side. Just make sure the board has cooled before disturbing it, or parts could be moved.
Jon -
Populate one side, bake, populate other side, bake. Usually surface tension is enough to hold the part on if its not too heavy.
Another option is reflow either both sides or just the second side with hot air, its fast enough that the heat does not reach the other side.
https://www.eevblog.com/forum/chat/how-do-companies-solder-double-sided-pcb_s-with-surface-mount-components/
https://www.eevblog.com/forum/manufacture/double-sided-reflow-at-home/
https://www.eevblog.com/forum/beginners/correct-soldering-for-double-sided-boards/ -
You can have a board offset from the surface. Simply putting bolts with nuts through the mounting holes is enough.
With soldering, it would be a two pass approach. Generally you want to use solder with different melting points. But for a home shop and small components, the same solder may be fine, components will be held with tension.
In this case start with the side with all the capacitors, reflow them. Place the other side, reflow both. Hope for the best and keep in mind that some components would go though reflow process twice, so heat sensitive components may require more thinking. But capacitors should be fine.
I would not do the both sides at once, you will get too many tombstones, especially if applying solder paste and placing components by hand.
Through hole components would still need manual soldering. -
Thank you everyone! Had no idea it was as easy as baking twice with an offset to keep the other side off the ground.
Now to buy an oven. -
The boards must be designed for double reflow, bottom side first with components that can be held in place by the surface tension.
https://documents.indium.com/qdynamo/download.php?docid=606 -
Generally you want to use solder with different melting points.
Does anyone actually do that outside of specific niche applications? I would assume that most contract assemblers use the same solder so they can use the same (or at least very similar) processes and profiles to apply and reflow it. I don't think there are enough alloy options that give sufficiently different melting points without being substantially different in other respects. The surface tension alone should be enough to hold most components in place on the bottom side during the second reflow. There are some rules of thumb about solder joint area vs component weight. Glue is also an option if surface tension alone isn't enough, but I don't think I've ever seen a double sided SMT board that had components glued on one side--just wave soldered boards with SMT parts on the soldered side, where glue is required. -
And with all that said, looking at your boards, hand soldering them is definitely very easy and probably the most sensible option here. I suggest you practice a bit your soldering skills - makes much more sense here IMHO.
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Components on both sides are not an issue for series production. However, the board will pass through the oven twice.
Design rules:
- the side to be populated first should contain components which will be either held in place by solder surface tension or glue during the second pass
- heavy and sensitive components will be placed on the other side and will do only one pass through the oven
So far, so good, but:
To avoid components' displacement during the second pass, make sure to reduce jerk to the board. So less coffee before work.
Design suggestion for 2 or 4 layer boards: Since components on both sides actually reduce routing options, the reduction in PCB size often isn't worth the higher effort during manufacture. Since PCBs get cheaper with higher numbers, it might even save money to go for a design with SMD on TOP and THT being soldered from BOTTOM.
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Does anyone actually do that outside of specific niche applications?
It is not common in mass production, for sure, but it happens. It obviously increases the price and tread more as a special order.
At the same time for manual and small batch assembly it may be beneficial to actually get different solder compositions. Same solder would be fine for passives, but big ICs may cause issues. -
Another way to do this, is to use a lower melting temperature solder paste on the side of the board done second.
So high temp paste first, low temp paste second (at a lower over temp).
The first side doesn't get reflowed a second time as it doesn't get hot enough to melt.
Could be an advantage if the second side has SMD connectors or other parts that melt easily. -
As much as I'd love more practice, I'm just not the best at it. I certainly can do it, but I want everything to look super clean and nice. So I bought an air gun instead of an oven. I'm going to see how well I can use one of those. Wish me luck.
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Am I correct that this is a mixed board with SMD and trough the board components?
If it is like that I would use hotair on the SMD and handsolder the throught the board components.
Its not a large amount of components on the board so i think this would be the simplest way.
Another option is to reflow one side and then hotair the SMD on the opposite side. -
The board is relatively small. Unless one needs to do 100s of it, I would consider and soldering. Relais and what looks like large inductiors are anyway a bit tricky with reflow, as large parts can slow down the heating locally.
If one wants to prcatice reflow - maybe do one side SMD parts with reflow and one the rest by hand. -
Where has the time gone? This is my 100th post, already:
As much as I'd love more practice, I'm just not the best at it. I certainly can do it, but I want everything to look super clean and nice. So I bought an air gun instead of an oven. I'm going to see how well I can use one of those. Wish me luck.
Don't beat yourself up, luck has nothing to do with it.
I bought a Weller WHP3000 a while ago, for casual use you could use a (single) ceran plate for ~25US$.
Order a small SMD stencil with your boards (should go for ~10US$ these days).
Apply solder paste, populate one side.
Put the board on the heating pad of your choice, heat up to 200°C (temperature varies, make a test. Please convert to Fahrenheit, if required.). The solder paste will turn from grey to glossy. Apply additional heat with the air gun, see the paste turn silver. Congratulations - you soldered a board. If done correctly, you will have a hard time telling the process from the result.
To understand what actually goes on, have a look at the SMD-paste's datasheet or at the recommended reflow profiles of the components.
Edit: I recently took some pictures of the process. Maybe they fit in here.
0: setting up the stencil
1: one screen shows the layout, the other the BOM. The tweezers have Teflon tips
2: paste on the board. Note that this doesn't have to be perfect. The key to reflow is the surface tension of the solder in combination with the solder mask
3: placement. Again, not perfect.
4: put on cold heater
5: that process went with a temperature setting of 300°C. The PCB will never see reach that temperature. Reflow happens at 215..230°C.
6: the result. Obvious items would be the off-center LED - they are highly susceptible to crosswinds. Upper middle shows unsoldered pads, where I didn't apply enough hot air.
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Hi harerod!
Thanks for sharing your method. So basically the reflow is done by the extra heat from a hot air gun. So than a basic Ebay preheater could do the job, right? Like this one: https://www.ebay.com/itm/353071403919?hash=item5234b2138f:g:HdAAAOSwow5etPky
However if the PCB should be put on a cold preheater (I guess because the heat shock must be avoided), than the cheap preheater could overheat the board during warming as they don't have a precise thermal control. Do you think the 100 bucks preheater worth a try? The Weller one you have is expensive for me now.
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Short answer: yes. You will be fine. Do some experiments, watch how the solder paste behaves.
Before I scored the WHP3000 from a fellow forum member, I used a 25€ single ceran plate. That plate stills sees daily use, albeit as my tea warmer. I never put the PCB directly on the ceran plate, but used a small sheet of aluminium. Just a bit of scrap metal from milling, say 100mm x 200mm x 3mm. That acts as a buffer and improves handling.
You saw the sheet of tin foil? That greatly improves performance, by reflecting the heat onto the PCB. You could reflow solder a PCB with that setup alone, but I find using hot air yielding better results.
As for hot air, anything goes. I bought my tool dead cheap at Ebay. Emphasis on dead - certain switch settings put line voltage on the case. Fixed that minor nuisance and never saw a reason to spend money on a more expensive tool.
I rarely reflow boards. The pictures show a piece of test equipment for the lab, so total number of one. The hot air is mostly used for rework on prototypes. -
Nice!
And if I order stencil, do you think the electropolishing is needed? They say (at JLCPCB) that when the pin spacing is less than 0,5mm then it is recommended to avoid the paste stuck in the apertures. In my next project I am having 0,5mm pin spacing, QFNs and TQFPs. -
I couldn't say anything about JLCPCB, except that their advertising makes me dizzy and therefore avoid them. Since my time is way more expensive than the cost of materials for prototyping, I stay with proven suppliers. I used to source from Beta Layout in the past, but their quality and service doesn't hold a candle compared to multi-circuit-boards.eu or Würth. If in doubt, ask the supplier and follow their recommendation.
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I couldn't say anything about JLCPCB, except that their advertising makes me dizzy and therefore avoid them. Since my time is way more expensive than the cost of materials for prototyping, I stay with proven suppliers. I used to source from Beta Layout in the past, but their quality and service doesn't hold a candle compared to multi-circuit-boards.eu or Würth. If in doubt, ask the supplier and follow their recommendation.
JLCPCB are proven. Many people on this forum order from them PCBs and assemblies. If you stay with “proven suppliers“ you probably get milked.