Continue to Site

Welcome to EDAboard.com

Welcome to our site! EDAboard.com is an international Electronics Discussion Forum focused on EDA software, circuits, schematics, books, theory, papers, asic, pld, 8051, DSP, Network, RF, Analog Design, PCB, Service Manuals... and a whole lot more! To participate you need to register. Registration is free. Click here to register now.

Register Log in

Resistor mounting conditions for full power rating at 70degC ambient?

Status
Not open for further replies.
T

treez

Guest
The following 2512 SMT resistor…..

PF2512FKF7W0R007L (7 milliohm)
https://www.farnell.com/datasheets/1563633.pdf

What is the mounting description (ie area of surrounding thermal copper pour) that corresponds to the rating of this resistor as being 100% full power (ie 1W) up to 70degC ambient?

The attached doesn’t shed much light either on what could be the rating of the above resistor when mounted on a crowded , hot PCB in 70degC ambient, in a non-vented enclosure, with the resistor not necessarily being mounted to make use of convection current cooling…and no thermal via’ing to bottom layer copper pour/heatsink.
 

Attachments

  • Chip resistor mounting.pdf
    559.7 KB · Views: 146

You might get the resistor working, but its going to kill its neighbours.
 
  • Like
Reactions: treez

    T

    Points: 2
    Helpful Answer Positive Rating
As I read the data sheet, the resistor has already reached its maximum working temperature/dissipation limit due to the PCB being at 70 *C, without any power actually being dissipated in the resistor. This means that when power is dissipated in it, its actual temperature will rise leading to a lower power rating. i.e. from fig 6, at 120*C, its power rating is only 50% of the 1W.
Frank
 
  • Like
Reactions: treez

    T

    Points: 2
    Helpful Answer Positive Rating
In the PDF in the top post, page 2 says…..
Heat conducted by the connections (of the resistor) must not reach the melting point of the solder at the joints. Therefore a maximum solder joint temperature of 110 °C is advised.
Click to expand...
Do you know what they mean saying solder melting point is 110degC?
 

That sounds like the internal joints of the resistor are soldered with a 110*C solder. This goes against their dissipation v temperature graph as the temperature must not exceed 110 not the 150 shown.
Frank
 
  • Like
Reactions: treez

    T

    Points: 2
    Helpful Answer Positive Rating
Solder goes soft well before it officially melts.
Think about how soft red hot steel is, its like chewing gum, and still well below melting point.

Its not unheard of to find a piece of equipment has stopped working and to discover a vital resistor completely missing from the circuit board.

Then to discover said resistor rattling around loose inside the case.
 
  • Like
Reactions: treez

    T

    Points: 2
    Helpful Answer Positive Rating
Its not unheard of to find a piece of equipment has stopped working and to discover a vital resistor completely missing from the circuit board.

Then to discover said resistor rattling around loose inside the case.
Click to expand...
..thanks Warpspeed, do you think that could happen as low as 110degC though?

We are just doing a PCB with a load of smps's on it and the internal ambient is 70degC and there are no air vents...there is a water cooled heatsink.......but it is above the PCB and we have to gap-pad the smt components to it.

Do you reckon we might need thru' hole components instead (instead of SMT), if our solder joints are going to get up to 110degC. I am not sure because all my "hot" smps's used thru' hole semi's and so i am not used to experiencing this with smt.
 

I would worry a lot more about SMT than through hole. At least with through hole you can mount your wire wound resistor on tubular ceramic stand off beads, to get some air around the thing.

Anything over about 80C will definitely turn your nice green fibreglass circuit board black over time.
blackdeath.jpg
 
  • Like
Reactions: treez

    T

    Points: 2
    Helpful Answer Positive Rating
Do you know at what temperature (temperature of the solder joints) that the SMT components (2512 resistors, QFN type FETs, little 20tssop controllers etc) can start to slide off the pads?
 

I do not know.

But I can tell you that normal "power" resistors are rated for typically 300C surface temperature rise at full rated power dissipation.
That is the industry standard believe it or not.
https://www.ohmite.com/techdata/res_select.pdf

.....And the official melting point of 60/40 solder is 183C.

I have never thought of ANY surface mount component as being a serious contender for high power.
 
  • Like
Reactions: treez

    T

    Points: 2
    Helpful Answer Positive Rating
Thanks, I am wondering if solder "suddenly" melts at 183 degC, or does it go 'soft' (like chewing gum) well before that?...say at 100degC?
 

Most metals go soft when heated and go "plastic".

Try hanging a spiral wound from resin cored solder, and heating it with a hot air gun.
See if it droops and sags under its own weight before it actually turns completely to liquid.
 
  • Like
Reactions: treez

    T

    Points: 2
    Helpful Answer Positive Rating
on a similar note, these SMT fets claim to be able to operate up to 150degC junction temp, but i surely there solder would go soft at 150degC and they could slide off the pads...
**broken link removed**
 

The junction may be at 150C, but its unlikely the lead temperatures reach that. The thermal resistance junction to case, and power dissipation should tell you.

Don't forget to de- rate the power as junction temperature rises.
 
  • Like
Reactions: treez

    T

    Points: 2
    Helpful Answer Positive Rating
The junction may be at 150C, but its unlikely the lead temperatures reach that. The thermal resistance junction to case, and power dissipation should tell you.
Click to expand...
The RTH(JA) is 1.5degc/w for that fet , so the pad temperatures wont be much less than 150degC...that si7370 fet is meant to cool via the pcb to bottom layer copper, then heetsink, so the 1.5degc/w applies to the route through its opads to the pcb....
I reckon it could slide about at 150degc of the junction, as the pads woudl be about 145degrees.......the solder woudl surely be soft.
 

That is impossibly hot, not only for the solder but the board. it will go black very quickly and probably de-laminate.
The mosfet may spit the dummy fairly quickly too.
 
  • Like
Reactions: treez

    T

    Points: 2
    Helpful Answer Positive Rating
on a similar note, these SMT fets claim to be able to operate up to 150degC junction temp, but i surely there solder would go soft at 150degC and they could slide off the pads...
Click to expand...

The solder used inside fet's etc is a much higher eutectic, typ 220C, 280C for RF xtors and some diodes

p.s. for your resistors, all components must be de-rated according to other heat sources around them and the heat-sinking afforded by local conditions, you cannot have a load of 2512 R's all doing max dissipation near each other and expect it to work, engineering is the art of sensible de-rating based on experience and thermal know-how and modelling...
 
  • Like
Reactions: treez

    T

    Points: 2
    Helpful Answer Positive Rating
Easy peasy said:
The solder used inside fet's etc is a much higher eutectic, typ 220C, 280C .
Click to expand...

The most common tin-lead eutectic has a temp of less than 200C (180C). The lead free solders are slightly higher but in the same range...
 
  • Like
Reactions: treez

    T

    Points: 2
    Helpful Answer Positive Rating
So, you are obviously not familiar with the solder types used inside the packages....
 

treez said:
if solder "suddenly" melts at 183 degC, or does it go 'soft' (like chewing gum) well before that?...say at 100degC?
Click to expand...

The melting is always sharp- but for some alloys and mixture. Iron is an exception - it has an extended plastic region. The mechanical strength gets reduced but it does not disappear.

The chewing gum state is called the "glassy" state (plastic; low strain) and is not common. The common solder will become soft and plastic around +/-2C of the melting point...
 

Status
Not open for further replies.

Similar threads

Part and Inventory Search

Welcome to EDABoard.com

Sponsor

Back
Top