-snip- lots of stuff
With regards to your comments about “upping yields” by adjusting solder deposits, are you saying that it’s not that critical on one offs? I am very inexperienced in this field and thought that too much solder paste on large thermal pads under IC’s may cause the chip to “float” causing problems at the pins and that this would happen no matter what the reflow process would be.
On the'lot's of stuff. That is exactly why the real machines used in industry use a 'static air mass'. THere is no air flow over the board ( or quasi none) the board is shoved in a pool of stagnant air that is held at a specific temperature. THe oven has multiple zones. the conveyor moves a t fixed speed and the 'time' the board remains at a certain temperature is determined by the length of the zone. Thats why these machines are 2 to 3 meters long ( Or more. I've seen reflow ovens that are 5 to 6 meters long ). They have an area where the air ( or nitrogen in some cases ) is heated and then flows down through a baffle. this creates a constant pool of air at a fixed temperature. There is virtually no airflow so no parts will blow away.
Now, on the one-offs : if the part doesn't 'flow or it floats you simply rework it. Not something you want to do in mass production. there you want the whole board correct in one shot , time after time. any rework is extra time and cost.
In mass production they tune the paste deposition and zone settings of the oven by running multiple boards ( sometimes hundreds ) till they got it right. On large boards they use sacrificial boards stuffed with thermocouples and a recorder that gets sent through the oven to see the profile at various points of the board. This allows them to fine-tune the whole thing.
if you are doing a one-off you are not going to sit there first on a bare board with thermocouples figuring that out. you wing it.
There is a perfect solution for one-off's though and that is using a vapor phase reflow oven. Those machines are guaranteed to produce a perfect soldering profile over and over but they come at a cost and their construction makes them difficult to use for mass production as they can only do 1 board at a time and do not have a conveyor ( experiments have been done but largely failed due to the physics involved in vapor phase soldering )
A vapor phase machine is basically a turkey fryer with a cooling ring around the top. You take a deep vessel and put a heating element at the bottom. Around the top of the vessel you put a couple of turns of hollow pipe where you pump a cold liquid through. Now , you pour a measure of liquid with a known evaporation point in the bottom and heat it up until you get a cloud of steam. This is where the physics kicks in. Water , at a standard atmospheric pressure of 1 bar boils at exactly 100 degrees C. Now matter how much heat you crank in that heating element at the bottom : the steam will always bee 100 degree C. you either raise pressure to increase boiling temperature , or you use a different liquid.
the vapor phase ovens used to use some nasty chemicals which were very toxic , especially when inhaled .... but since the creation of' Liquid Teflon' in the mid 80'2 this problem has been solved. The chemical used is called Galden and made by Solvayt. Galden is a totally inert liquid that boils around 190 degrees to 240 degrees depending on the formulation you buy. Galden is available in temperature grades . Steps are about 3 degrees C. You can drink that stuff, it passes straight through your body. Can't be absorbed , can't be destroyed by acid. It is basically a PTFE just like Teflon.
So what do we do : we heat a puddle of Galden until it evaporates in a cloud at a static temperature (determined by the physics) the heat makes the cloud rise. as it approaches the cooling zone it drops below vapor phase and condenses on the cooling rings and drops back down into the puddle of liquid where it is re-heated.
So you essentially make a heat-pump using a cloud of vapor.
Now all you have to do is lower your board in the CLOUD ( not in the liquid ! ). since the board is cold at first the Galden condenses on the board and drips back off the bottom but int the process it disposes of its heat into the board. The board and parts act as heat sink. When the board has fully warmed up to the temperature of the Galden cloud, the Galden evaporates off the board as well. there is no risk in overheating as the cloud temperature is a constant. all you have to do is lower the board in the cloud : see it get 'wet' with Galden , wait for the board to become dry at which point all solder becomes liquid and then gently lift it out of the cloud , through the cooling zone. done
Galden is eutectic in nature just like solder. It toggles from liquid to vapor at 1 and only 1 fixed temperature. There is no 'zone'. it is a very 'thin' liquid, it flows better than water and has a very low molecular weight. This makes sure that a puddle of Galden cannot push a part aside. it simply doesn't weight enough. It is a very strange liquid. It is also used as a coolant in various high-tech applications ( a coolant is basically a heat transfer liquid ) like ion implanters where it replaces freon. When i was in working in the waferfab we had to stop using freon and switch to Galden. I remember the little weird 'balls' it was packaged in. Like a cube but with rounded edges and a spout on the side. Bloody expensive stuff. probably has come down in price now - nope just checked. Still at well over 1000$ a bottle ) http://www.pchemlabs.com/subcatagoryb.asp?PID=Galden-Heat-Transfer-Fluids
So these machines produce consistent soldering results
problem is :
- one board a time and you need to wait for the cycle to complete
- time to lower and raise the board
- you contaminate the Galden with flux remains.... but you can 'scrub' the Galden to regenerate
- Galden is an extremely expensive liquid. To the cost of over 1000$ per bottle. You may as well use mouse-milk ...
These ovens are used a lot in the manufacturing of military , avionics , space and medical devices. simply because of the excellent process control and 'endpoint' detection ( board dry = solder liquid without risk of overheating. there is also no problem with oxidation once the flux has evaporated as the whole board is submerged in a cloud of Galden. There is no Oxygen around... so the solder or copper does not oxidize once flux has evaporated.http://www.asscon.de/e/pages/products/laboratory.htmlhttp://www.lesker.com/newweb/fluids/heattransfer_galden_ht.cfm?pgid=0http://www.solvayplastics.com/sites/solvayplastics/EN/specialty_polymers/Fluorinated_Fluids/Pages/Galden_PFPE.aspx