Reflow Oven preheating (Not board preheating)

[washoegary]

I have viewed numerous Re-flow Oven (Reviews-Techniques-Tips etc.) videos and came across a re-flow tutorial where the oven was pre-heated to the soak temperature before the room-temperature PCB was placed into the oven for re-flow. The oven was a low-end commercial model manufactured by Manncorp (Makers of commercial SMD equipment). I am curious what anyone's thoughts would be on using this pre-heating technique?

Video: 850 Desktop Reflow Oven Tutorial: https://youtu.be/AE12KZXELjw?

[helius]

The most even heating profile requires convection. Preheating the oven could help with reaching a uniform convection air temperature and reducing the contribution of IR heating which can cause scorching.

[wraper]

Preheating is important if you want stable results. Especially if there are parts with very different thermal mass. But I decided to gave up on IR (my oven also contains a fan which is moving the air inside, which helps somewhat) and ordered a vapor phase oven recently. Too much pain in the ass with uneven heating, and when you need to reflow some bulky metal body connectors, you a scrotching the rest of the board while those are not even started to reflow. Hard to get it right, especially when doing different boards.

[mrpackethead]

Preheating is important if you want stable results. Especially if there are parts with very different thermal mass. But I decided to gave up on IR (my oven also contains a fan which is moving the air inside, which helps somewhat) and ordered a vapor phase oven recently. Too much pain in the ass with uneven heating, and when you need to reflow some bulky metal body connectors, you a scrotching the rest of the board while those are not even started to reflow. Hard to get it right, especially when doing different boards.

Which Vapour Phase oven did you get?

[washoegary]

Preheating is important if you want stable results. Especially if there are parts with very different thermal mass. But I decided to gave up on IR (my oven also contains a fan which is moving the air inside, which helps somewhat) and ordered a vapor phase oven recently. Too much pain in the ass with uneven heating, and when you need to reflow some bulky metal body connectors, you a scrotching the rest of the board while those are not even started to reflow. Hard to get it right, especially when doing different boards.

Thanks for the tip. I need to check into vapor phase ovens.

[marshallh]

Preheating is important if you want stable results. Especially if there are parts with very different thermal mass. But I decided to gave up on IR (my oven also contains a fan which is moving the air inside, which helps somewhat) and ordered a vapor phase oven recently. Too much pain in the ass with uneven heating, and when you need to reflow some bulky metal body connectors, you a scrotching the rest of the board while those are not even started to reflow. Hard to get it right, especially when doing different boards.

do you have IR heat shields over the elements?

[wraper]

do you have IR heat shields over the elements?

I made not exactly shields but reflectors so the IR is reflected to the parts  which are furthest from the lamps. There is almost no direct IR reaching the board. By scorching I meant is not exactly "scorching" as the boards are working but some parts certainly can get overheated on some bigger boards. I tested it with analyzing the image of the boards taken by thermal camera.

Which Vapour Phase oven did you get?

Imdes mini condens IT for EUR 700 (+EUR 55 shipping) but I have not received it yet. I also ordered 2x 0.5l bottles of Galden LS230. Each bottle cost me EUR 145. Oven needs only 300-350ml but I decided that I don't want to search for this stuff in a few years, as it can become very tricky (and expensive) to get.

[JuKu]

I didn't watch the video, but in general: You don't want to heat the board too fast, and you want to ramp up the board temperature evenly. I fail to see how preheating the oven helps in that. Sounds to me that this promotes fast, uneven heating of the board, where sections of the board that are closer to hot parts of the oven heat far faster than parts of the board protected by heavy components. This is a recipe for microfractures on the board, which are a nightmare to find and debug.

Correct pre-heating and soaking is also essential for the paste to operate as designed; look at your paste manufactures data sheet. Sounds to me that putting a board in an already hot oven more or less guarantees out of spec conditions for the paste, at least in some areas of the board.

Summary: At first hearing, sounds like a rather bad idea. I'd like to hear the motivation behind it.

[Kjelt]

I agree with Juku and others that this is not a good idea.
My guess is that it probably is only to coverup the bad design of the oven, which probably is not able to follow the correct soldering reflow profile (<1,8^oC/second).

The biggest problem you will see is temperature shock of components and that the paste might act up violently.
The flux in the paste starts to become really fluid and this should be slowly at the temperatures below the first stop at around 160 degrees C.
If this goes too fast it might displace the components (tombstoning or misplacement).

I just now see that Wiki already has a nice article on it:
https://en.wikipedia.org/wiki/Reflow_soldering#Preheat_zone

Preheat zone
Maximum slope is a temperature/time relationship that measures how fast the temperature on the printed circuit board changes. The preheat zone is often the lengthiest of the zones and often establishes the ramp-rate.[1] The ramp–up rate is usually somewhere between 1.0 °C and 3.0 °C per second, often falling between 2.0 °C and 3.0 °C (4 °F to 5 °F) per second. If the rate exceeds the maximum slope, damage to components from thermal shock or cracking can occur. Solder paste can also have a spattering effect. The preheat section is where the solvent in the paste begins to evaporate, and if the rise rate (or temperature level) is too low, evaporation of flux volatiles is incomplete.

[IconicPCB]

Preheating is essential and a normal part of oven operation...


consider an oven whihc has been used to solder a few boards in succession is it any less preheated than an oven whihc has done no work and only been switched on empty?

Why should the fourth board experience a hot oven and not whinge compared to the first board whihc will suffer death and destruction because the oven has been preheated?

[Kjelt]

Because good reflow ovens force a cool down of the oven back to room temperature see any reflow profile.
And the pro industrial ovens have conveyor belts and multiple heating zones that follow the reflow profile.

[mjkuwp]

Preheating is essential and a normal part of oven operation...


consider an oven whihc has been used to solder a few boards in succession is it any less preheated than an oven whihc has done no work and only been switched on empty?

Why should the fourth board experience a hot oven and not whinge compared to the first board whihc will suffer death and destruction because the oven has been preheated?

yes, it could be a problem.

The way I did mine is that my Start button is not allowed to start unless the feedback temperature is under 30C.  I have an auto-start trigger that begins a ramp when a 10% duty cycle of the oven brings the oven temperature to 40C AND when the slope of the temperature at that point is within a range that I specified.   My intent is to make sure the oven starts relatively cold and that the temperature is rising at the same rate as it crosses 40C for each run.

This has worked well.  By the time I finish doing any manual thru-hole soldering and correct any solder bridges, etc. the oven is cold enough for the next board or batch.

my thought is a 'no' on preheating the oven before putting the board in.

[IconicPCB]

Consider the ovens "big boys" play with.

Thay the ovens are not batch ovens, they are conveyorised ovens typically.


They have a preheat zone a soak zone followed by a reflow zone and a cooling zone.

The preheat zone is brought up to operating temperature and maintained at the temperature as are all he other zones.

A product is then inserted into the process.

Consider the cheap and not so thermally capable batch ovens used by the hoby fraternity. Often times a very light construction, thermally lossy and thermally very short time constant.

Being a batch oven the temperature profile of the product being assembled is controlled in time by controlling power input into the oven as opposed to maintaining zones at set point and passing product from zone to zone.

Consider a board has just been soldered and removed from the batch oven. Temperature profile is no longer active, heaters are no longer energised. The oven temperature will start to reduce even if the oven is closed.
Certainly the oven thermal capacity is low and will not support oven maintaining set point.

Next board arrives a minute or so later. The oven is opened and in doing so it looses heat. A new cold board is inserted and it starts soaking up heat from the oven.
Oven controller steps in and says weeellll may be i dont need to do much right now the oven is at 150C  and it goes off and plays catchup waiting for the oven to come down a bit before it says time to get going.

So the product is exposed to a higher than planned temperature. Bad Juju.
But hang on in a commercial big boys oven preheat zone is up in the 150 C area in the preheat zone.

How is that good and a tiny batch oven with low thermal mass and probably no convection assist is going to crack ceramics and over stress microwire joints inside semiconductor packaging.




 

[wraper]

Preheating is essential and a normal part of oven operation...


consider an oven whihc has been used to solder a few boards in succession is it any less preheated than an oven whihc has done no work and only been switched on empty?

Why should the fourth board experience a hot oven and not whinge compared to the first board whihc will suffer death and destruction because the oven has been preheated?

yes, it could be a problem.

The way I did mine is that my Start button is not allowed to start unless the feedback temperature is under 30C.  I have an auto-start trigger that begins a ramp when a 10% duty cycle of the oven brings the oven temperature to 40C AND when the slope of the temperature at that point is within a range that I specified.   My intent is to make sure the oven starts relatively cold and that the temperature is rising at the same rate as it crosses 40C for each run.

This has worked well.  By the time I finish doing any manual thru-hole soldering and correct any solder bridges, etc. the oven is cold enough for the next board or batch.

my thought is a 'no' on preheating the oven before putting the board in.

As for me, such oven would fly right out of the window. I will not wait half an hour until the oven cools down below 30^oC so you can solder a next board  . BTW what you are doing in the hot summer day, does it require air conditioner to start?