Calculating PSI from Axial Force
Calculating PSI from Axial Force
(OP)
We have a heatsink .86 x 1.86 x .125, Al 6061, We are using 440 cap head screws to attach. The screws are .200 from each end, only two. The thermal conductive material is T805 and requires 100 PSI to achieve its thermal conductivity value. Thatcalculates to 160 lbs of force over the 1.6 sq in surface. I can caluclate the axial force of the screws, however I need to determine the PSI distribution over the entire interface, any questions directions or suggestions is appreciated.
Chauvet
Chauvet





RE: Calculating PSI from Axial Force
what is the thickness of the adhesive?
what is its E?
what are the beam properties of the heatsink?
Cheers
Greg Locock
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RE: Calculating PSI from Axial Force
what is your best guess approach?
which university did you go to?
Cheers
Greg Locock
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RE: Calculating PSI from Axial Force
Chauvet
RE: Calculating PSI from Axial Force
Regards
RE: Calculating PSI from Axial Force
RE: Calculating PSI from Axial Force
ht
RE: Calculating PSI from Axial Force
But you have to know how to calibrate the film (don't look at me! We got ours done at National Research Council-Canada). As I suppose it would be difficult to calibrate the rather ingenious method (0.25 thick Al) you came up with--you can see there is an impression made on the Al, but what is the pressure level? You can only guess.
I don't understand where the pressure is coming from. Is this magical 100 PSI that you need the pressure you see in the middle between the two objects you've screwed together (the HS and whatever the base is)? If so, what the pressure distribution looks like, the shape of it, depends almost entirely on the shape of the two objects being pressed together. Say your two surfaces are perfectly flat, I would expect the pressure distribution to be flat, or constant. What if one is perfectly flat, and the other is a little bit curved, AWAY from the flat surface, so the two bodies are touching at the edges, but not in the center (the opposite of a ball being pressed onto a flat plate, that is). Then you could have ZERO pressure in the middle if the two objects aren't in contact in the middle.
RE: Calculating PSI from Axial Force
http://www.tekscan.com/index.html
Yet another physical method is using ultrasound - here is a technical journal article on this method:
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You also could use computer software to calculate the answer. For example, Finite Element Method with contact elements. This is probably your best bet.
If you want a standard analysis method with equations, I think you will be disappointed. The fastener clamping force creates a ring of pressure around the fastener hole, but the magnitude is a function of so many variables that it isn't modeled with basic equations. Your instincts are leading you in the right direction - relying on two screws at the outer edges isn't going to be a good guarantee of high clamping force in the middle.
Regards,
Cory
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RE: Calculating PSI from Axial Force
I like the idea of the pressure film. I'd probably use FEA.
So far as equations go, what you have approximately is a simply supported beam on an elastic foundation.
The good news is that that is a solvable case, for uniform beams.
The bad news is that there is no simple general analytical solution for beams of varying width, which is what you have.
If you measure the contact pressure then you could use the standard beam equation to work out how much to pre-bend the heat sink before installing it.
Cheers
Greg Locock
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RE: Calculating PSI from Axial Force
RE: Calculating PSI from Axial Force
RE: Calculating PSI from Axial Force
Sincerely appreicate the responses. Bare with me as I respond to each. TheTick, yes it would if I had a uniform pressure over the entire surface, I need 100 psi, I have 1.6 sq in therefore I would need 160 lbs of force, however I have only the two screws to apply force. CoryPad, thank you, may I ask the source?. The rule appears to be the same, and is resonable, as with Thermal distribution in a fixed ambient temperature. I will use your pictorial. It is the math that I am out of the league with. Will investigate the journal. GregL, yup, this is a simple real world problem, I will investigate your recommendations and I totally understand the pre-stressed beam example, however that is too elegant a solution for this bunch. Prost and UncleSyd, familiar with the Fuji material, trying to get some, however time is my nemesis. Prost, the object is a rectangular Al HS, .86 x 1.86 x .125 (original design) the graphite film is a TC film that transfers heat, the HS has a 32 RMS finish with Ni plate and is .0004/in across the HS. The 100psi is specified by the Thermagon folks to achieve the 5w/mK (Thermal conductivity of the graphite film). The issue is that any pressure less than the 100 psi, the thermal resistance goes up (inverse of thermal conductivity). The T805 material is compressible under the screws so the HS bows and there is no contact in the middle, (many flatness experiments)between the AL and T805, and air is a poor thermal transfer medium. I progressed up to .250 thick Al and can get a contact impression (see previous notes)across the entire film surface, Although this does not guarantee the, all important, pressure in the middle of the HS, therefore, Strain gauges and Fuji film are my next shots. My further recommendation will be to put two screws nearer the center, total of 4 screws across the length of the HS, the complication is there is a 50W power amplifier mounted in the middle of the HS. Will send pics of the WHOLE problem as time permits tomorrow. I genuiely appreicate all of the responses. Anu more thoughts are appreicated.
Regards
Chauvet (not a handle, the real name)
RE: Calculating PSI from Axial Force
If space limitations dictate that it can't be thicker, perhaps a dab of thermal grease and no spacer would do beter?
Mike Halloran
Pembroke Pines, FL, USA
RE: Calculating PSI from Axial Force
That is what the two screws were for, however we will have to add screws with increased area, the problem still lies wit the circuit board layout on top of the heat sink and room for the addtional screws.........
RE: Calculating PSI from Axial Force
Mike Halloran
Pembroke Pines, FL, USA
RE: Calculating PSI from Axial Force
Regards,
Cory
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RE: Calculating PSI from Axial Force
Regards
CP
RE: Calculating PSI from Axial Force
Cheers
Greg Locock
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RE: Calculating PSI from Axial Force
I'm with Greg Locock, machine a profile.
RE: Calculating PSI from Axial Force
Any comments on yes this is true or no and here's why is apprecaited, It just seems that the spacing between the screws is a factor.
RE: Calculating PSI from Axial Force
Regards,
Cory
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RE: Calculating PSI from Axial Force
RE: Calculating PSI from Axial Force
I think if you have enough screws to supply the required force, then your biggest problems will be, as stated previously, bending of the heatsink and manufacturing tolerances on the components.
RE: Calculating PSI from Axial Force
AlBigger, you are correct, many HS manufacturers prestress or dimple the contact area for improved contact. The screw problem is the pressing one for me. The boys that designed this module dont want to change the design and add screws, they want me to tell them what the pressure is in the center of the heat sink with only two outer screws. They assume that we can get there with more torque and thicker matl, that is correct however you have 500 lbs of force on the ends and 150 in the center and the long term reliability of the joints come into question. I found the Handbook of Bolts and Bolted Joints and some software that is suppose to actually solve the very porblem I have, those with the Fuji Film that is suppose to arrive today, sould get me there.....will keep a light in the window until the problem is solved.
C
RE: Calculating PSI from Axial Force
RE: Calculating PSI from Axial Force
Which means it's not a simple beam on elastic foundation problem anymore. It's a composite beam of nonuniform section, and you may not know enough of the package details to analyze it correctly.
My head is starting to hurt.
At this point, I'd be reading Freescale app notes and spec sheets, and doing exactly as they specify.
Mike Halloran
Pembroke Pines, FL, USA
RE: Calculating PSI from Axial Force
http
Until you know the stiffness of the thermal paste all bets are off.
Cheers
Greg Locock
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RE: Calculating PSI from Axial Force
SREID: The phase change materials are superior for gap filling, they do not have as good a TC, however due to the very thin (.003)and gap filling abilities they have outstanding LOW thermal resistivity. Unfortuately I am told to "stick" with T805 for rework and ease of cleanup.
Mike: WELCOME TO MY WORLD...............I am ex Moto and I have the APP notes pasted to the cubicle wall. They note two 440 screws @ 5in lb offer 450 lbs of force. The device is .2 sqin in size. They use T805 undere the device, however we solder it down. Yes, not only is there a bending moment with the package, then there is a PCB around it and the PCB is soldered to the HS.
GregL: thanks for the info...... (The pictures do not come through, will attempt to contact you via your home page.) They are using Thermal Paste in the lab, everyone is a thermal genius in there, So I let them do what they want, I am focused on the shippable product.
I received a sample of the Prescale FugiFilm, interesting stuff. It works, however like statistics you can make it do what you want, to an extent. The imprints simply do not show the pressure we expected in the center of the HS. I used the sample without the T805 between the two HS blocks, the argument here which is correct? I do get two different, however small,impressions if I use the T805 material or not. Any experience out there on the Fugi material?
RE: Calculating PSI from Axial Force
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Cheers
Greg Locock
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