What is delta t?

#52164

What is delta t? | 24 October, 2007

hi everyone I'm new at forum. Could someone explain me what is delta t and why its so important.

thanks

marcin

#52165

What is delta t? | 24 October, 2007

It means "difference in temperature". It's important so you know when to put your sweater on or take it off.

Dirk Nuendyke

#52168

What is delta t? | 24 October, 2007

To expound on what Hussman says, Wikipedia defines Delta (a Greek letter that looks like a triangle) as, "A macroscopic change in the value of a variable in mathematics or science." Anyone who's taken Physics would know what this is.

In soldering, it's important that you don't go from cold-to-hot too fast - hence, the term "delta T" (change-in-temperature) that you hear the soldering guys at your company talk about. I betcha them soldering guys get BLAMED for everything too, right? :-)

#52171

What is delta t? | 24 October, 2007

Actually, it applies to conditions from cold to hot AND hot to cold. The measure of T is graphed against time. Time is what you measure the difference of temperature against. I was hoping to influence this engineer to find this out on his own, but.................

Steve Thomas

#52173

What is delta t? | 24 October, 2007

Actually I didn't see anything saying this person was an engineer, but I digress....

In this case this person may very well be referring to the delta t across space rather than over time, as in different locations on a pcb that's being profiled.

#52182

What is delta t? | 24 October, 2007

I was asking about delta t in profiling, because I have just started to learn how to do it and wanted to know about it. So how can you reduce delta t, so you dont go from cold to hot too fats. Is it enough to slow donw conveyor speed???

Thanks Marcin engineer to be

#52185

What is delta t? | 24 October, 2007

Good points on both subjects. Not all who talk engineer speak are engineers. It depends on how shiny their shoes are. Delta T across any medium can mean a myriad of different set values. From Polynomial Expressions to the names of any organization that wants a �smart� name. I thought I knew my audience and took a stab at it. Once again the old age adage of helpful kindness will often be rewarded with the Teamwork Dagger.

#52187

What is delta t? | 24 October, 2007

Ah, see Steve, I was right. It depends Marcin. If you use the same temps in each zone for all your product, vary you converyor speed. This will negate any waiting time between profiles. Slowing the speed will decrease your Delta-T to an acceptable time specified by your component supplier.

Steve Thomas

#52188

What is delta t? | 24 October, 2007

You may very well be right Hussman, but I'm not certain that marcin knows exactly what he wants to know. :) I would call that ramp rate, not delta t, but that's my language.

marcin, what I'm talking about are the varying temperatures on your board due to the mass of each component and their ability to absorb heat. An 0603 all by itself will get hotter faster than one of say, 4 QFP 240s laid out in a tight grid (not such a great idea but this isn't the design forum so we can talk about such travesties), so your delta t at any given time will be greater than on a board with just resistors, caps, and SOIC8s.

Slowing the conveyor does give the big parts time to catch up, but you do have to reduce your heater output to avoid excessive peaks, time above liquidus, etc.

#52191

What is delta t? | 24 October, 2007

Ramp rate is measured in degrees per second. Max and min temps are not required to to establish "ramp rate". The difference between parts, like I was describing is Delta T. But, ramp rate is a generation from this as well. Most component manufacturers talk about "max temp" and I believe that is what Marcin is probably working with here. Now establishing your proper reflow temps on every component with varying ramp rates that produce different delta-t across your board is a fine balancing act between conveoyor and heat source. That's where the engineering part of the job come in.

Dirk Nuendyke

#52196

What is delta t? | 25 October, 2007

Forget all the bickering and semantics. EVERYONE IS RIGHT!! ...but then again, in soldering, there IS no right, wrong, or indifferent... just blame. :-)

Soldering is, after all, the most difficult process with the most opportunities for defects...and...and...the easiest process to blame. Problem is, not everyone understands soldering. It's a complex, scientific, process requiring some rundamentary knowledge of Physics, Chemistry and Metallurgy, and also knowledge of machines and how to "finesse" your machines to make bright shiney solder joints... NOT SHINEY SHOES, mind you...SOLDER JOINTS.

Anywho... let's get back to semantics. Without googling or Wikipedia, here is what delta T is as I know it.... :-)

Delta T, as Hussman so eloquently states, is the temperature DIFFERENCE (delta) from one point in time to another. ...and this could mean COLD-TO-HOT OR HOT-TO-COLD.

Thermal Gradients, on the other hand, are the temperature differences across components and your Printed Circuit Board assembly as they go through your heat-induced processes like Reflow and Wave Solder. Thermal gradients are different across an assembly due to thermal mass of your components, and hence different heat conduction rates. Reducing the thermal gradients depends on how efficient your Reflow Oven is at transferring heat uniformly across your assembly. For example, forced convection is known to heat an assembly uniformly and is not sensitive to thermal mass or bright and dark surfaces, like say, Infrared preheat. Thermal gradients may also be reduced through thermal profiling.

DELTA T... let's get back to this now, shall we? In reflow, delta T is the temperature difference, your DELTA, from when your assembly goes from ambient, to the first zone of your oven, and then from your last zone to your cooling medium. In reflow, we often speak more of ramp rates, which are derived from delta T's. Basically, your ramp rate is your Change in Temperature/Change in Time.

DELTA T in WAVE soldering is the temperature difference between your last preheat, to when your PCB assembly hits the wave solder. Rule of thumb in wave soldering...reduce your Delta T as much as possible before it hits the molten solder. Rule of thumb is 100�C. This can be accomplished from either increase of preheat, decrease of conveyor speed, or reduction of solder pot temperture, or a combination of all. Delta T was made famous by the ceramic cap manufacturers and it was they who came up with that 100�C, and it is they who love blaming the wave when components crack due to thermal shock.

So there it is...semantics. EVERYONE IN THIS POST IS RIGHT AND NOBODY IS WRONG. :-)

#52197

What is delta t? | 25 October, 2007

Thanks for all your answers.

Could you guys tell me how you profile new job. I mean if you could how do you begin setting up profile for brand new job and so on. It would be very usefull for me.

Thanks MArcin

Paul Richardson

#52202

What is delta t? | 25 October, 2007

I can provide you with a Dundas chart to tell you delta Tea.

#52203

What is delta t? | 25 October, 2007

OK if you could email it to me it would be great

#52208

What is delta t? | 26 October, 2007

No, you're wrong! All that matters is you just wear nice shiny shoes, a long sleeve shirt and make sure the Delta-T in the crease of your pants doesn't exceed the ramp rate of your buzzwords. When people ask you very simple technical questions you cannot possible answer, just smile. Steal credit of others and always ingratiate yourself into others work without actually contributing. Always remember to lie-lie-lie-lie on your resume and BINGO! Todays new engineer has arrived.

Steve Thomas

#52212

What is delta t? | 26 October, 2007

Rather than go into detail here, try googling "classic reflow profiling".

One of the guys at ECD (a thermal profiler manufacturer) wrote a pretty decent article that covers most if not all of it.

You DO have a profiler, don't you, or an oven with the capability (software, thermocouples, thermocouple attachments)?

*waiting for Hussman's daggers* ;)