How to "toast" double-side PCBs on these conveyor ovens? - Page 1
Forum section: electronics / general

author: olkipukki
author_rank: Frequent Contributor
timestamp: February 24, 2019, 09:26:54 am
content: I am looking into small conveyor reflow ovens and trying to solve a puzzle how to reflow components on both sides.Is there any trick how to do this?

author: SMTech
author_rank: Frequent Contributor
timestamp: February 24, 2019, 02:04:35 pm
content: When reflowing the 2nd side you need something to lift it of the mesh conveyor bed, if you have tooling strip or space round the edges on your board the simple solution is to strap titanium guides used in wave soldering like thesehttps://www.grovesales.co.uk/titanium-strips/?___SID=Uto the sides. Other solutions could work but in small benchtop ovens like the ones pictured you would need to be careful your fixture didn't interfere with the reflow prcoess too much. On larger commercial ovens you would have a pin edge conveyor instead. You also need to make sure your oven has the height to clear a double sided board.

author: IconicPCB
author_rank: Super Contributor
timestamp: February 25, 2019, 01:15:05 am
content: It also goes without saying that a dollop of adhesive may be in order depending on component mass and PCB surface to prevent components dropping off.I typically solder the side with lighter cubic ( jelly bean ) components first and then pick and place the heavier components side.Typically i make up a carrier which shields the already soldered side from excessive temperature and then solder the second side.The shield is made from FR4 laminations.

author: Psi
author_rank: Super Contributor
timestamp: February 25, 2019, 02:14:29 am
content: Also, just in case it was not clear from what others have said above.The PCB goes through the conveyor reflow oven twice, e.g. each side is reflowed separately.Once a side has been soldered the solders surface tension will hold the parts to the PCB when going through the second time upside down.If large/heavy parts are on one side then this side is done last so it never goes through upside down.If both sides have large/heavy parts then glue dots are sometimes used to hold heavy parts to the PCB and stop them falling off.The bottom side of the PCB is not soldered while going through the machine upside down.If parts are on the bottom side when going through the machine then they have already been soldered previously.Only molten solder has enough surface tension to hold the parts on when upside down. Solder paste does not, they would all fall off or tombstone if you tried to do it.(It might be possible to glue every part to the underside and then solder both sides at once, but this isn't normal practice.)

author: mrpackethead
author_rank: Super Contributor
timestamp: February 25, 2019, 02:55:20 am
content: the surface tension of the solder will hold a certain amount of component weight. A good rule to work to is .5g / cm2 of solder paste. If you have more than that then you'll need to use an adhesive or make sure its on the 2nd side.

author: jmelson
author_rank: Super Contributor
timestamp: February 26, 2019, 10:10:59 pm
content: Quote from: IconicPCB on February 25, 2019, 01:15:05 amI do a lot of double-sided boards in a toaster oven, with thermocouple controller. One board has a big voltage regulator on the bottom (LM2575 switching regulator, 5-lead D-PACK.) I use no shield, and the regulator NEVER has any problem when I reflow the top-side of the board. The ONLY problem I have ever had is when I tried to take the board out of the oven before the solder cooled, and bumped a few parts out of place.Jon

author: mrpackethead
author_rank: Super Contributor
timestamp: February 26, 2019, 11:56:17 pm
content: Quote from: jmelson on February 26, 2019, 10:10:59 pmWhat is the weight of the regulator, and how much surface area does it have that has solder paste applied Jon?

author: olkipukki
author_rank: Frequent Contributor
timestamp: February 27, 2019, 11:57:58 am
content: Quote from: SMTech on February 24, 2019, 02:04:35 pmDoes that mean this board fixture will travel together with PCB board thought oven?Well, it should be a strong enough to hold a board in place (aka vibration etc.)In my understanding, proper commercial conveyor ovens have rails, is it correct?I am not sure right now if whole idea to replace a bench drawer oven with a basic conveyor will work without major headaches.My thoughts were I could connect an exhaust system to similar above ovens and have clean air in a lab, rather than open a drawer every time reflow more boards. As result breathe some fume and leave a smell around.What about a solder paste? Would you prefer (if you can) use a low temperature one for 2nd side reflow?

author: SMTech
author_rank: Frequent Contributor
timestamp: February 27, 2019, 12:42:30 pm
content: Quote from: olkipukki on February 27, 2019, 11:57:58 amquoteQuoteThey use a Pin edge conveyor, which is a chain with lots of fingers on it that hold your PCB by the edges, they might back this up with a center support rail to help with larger boards and sometimes have a mash conveyor as well. Not everything works with pin edge, although that's usually a design issue.QuoteNo, same paste both sides. There shouldn't be any major headaches but now getting your profile right will involve changing the temperatures and the conveyor speed to dial in on something good.

author: olkipukki
author_rank: Frequent Contributor
timestamp: February 27, 2019, 01:58:42 pm
content: Quote from: SMTech on February 27, 2019, 12:42:30 pmI've got it now, many thanks.Wondering if I will able to see such examples in Productronica...

author: SMTech
author_rank: Frequent Contributor
timestamp: February 27, 2019, 02:20:50 pm
content: I wouldn't be surprised, you might even see an entire line powered up and running...

author: jmelson
author_rank: Super Contributor
timestamp: February 27, 2019, 08:34:08 pm
content: Quote from: mrpackethead on February 26, 2019, 11:56:17 pmWell, it is a D-Pack, sort of like a TO-220 with the tab mostly cut off. I don't know the weight, but it must be 20 g or more - it is on a copper tab. No weight is shown on the datasheet.Now, the part is fully soldered first when the back side of the board is reflowed, so it is flush to the surface with maybe half a cm^2 on the tab, plus 5 leads.Then, when reflowed for the front side, it is just the solder surface tension holding it on the back. Never had a problem.Jon

author: HalFET
author_rank: Frequent Contributor
timestamp: March 01, 2019, 09:50:27 am
content: The professional solution is called a "SMT process carrier", and it'll help keep the components in place if designed properly and also limits board warp and other funny business. But to be honest, what I usually ended up doing was to stick a few ceramics plates underneath to raise it off the conveyor and a dab of quick-curing epoxy to keep the heavier components in place.

author: mrpackethead
author_rank: Super Contributor
timestamp: March 01, 2019, 10:00:20 am
content: I looked it up its 0.16ounces, or 4.5g

author: SMTech
author_rank: Frequent Contributor
timestamp: March 01, 2019, 10:41:25 am
content: Process carriers are more likely to be used when you are reflowing something sensitive or difficult to handle that needs support or protection. Think things like Flex or Rigid-Flex circuits, the flexible substrate doesn't like getting hot and also needs support and are often custom pieces of tooling.You do also get adjustable more generic setups which might hold the board through the entire process for reasons such as warping or because for some reason there is no support for tooling strip to make the board compatible with the 3mm clearance a belt would need, or its a very funny shape.Other custom carriers can be used to protect certain parts of the board when it is selectively or wave soldered.

author: HalFET
author_rank: Frequent Contributor
timestamp: March 01, 2019, 11:27:04 am
content: Quote from: SMTech on March 01, 2019, 10:41:25 amI entirely agree with you, that's what we used them for 99% of the time. Though for single-sided flexible circuit boards I do like the tacsil-tape stuff from Taconic a bit more since it guarantees dimensional stability to a higher degree. But that aside, it is convenient to use simple SMT process carriers if you're going to assemble a large series of standard boards with components that don't "stick", a properly designed carrier can hold them in place without the need for adhesives. The only issue with making cavities in them like that to support components is that you got to be careful when using them in conjunction with a vapour phase oven.

author: olkipukki
author_rank: Frequent Contributor
timestamp: November 15, 2019, 06:29:42 pm
content: Quote from: SMTech on February 27, 2019, 02:20:50 pmMore or less, a live demo-simulation factory line with a dozen or so machines lined up as a long train.A low 7 figures $ and its' yours.

author: Pseudobyte
author_rank: Frequent Contributor
timestamp: November 15, 2019, 10:17:33 pm
content: We just use stock FR4 cut into sticks. You can buy it on McMaster Carr.