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SMT electronics assembly manufacturing forum.


Thermally Conductive Epoxy

JohnW

#7526

Thermally Conductive Epoxy | 1 August, 2001

hi Folk's,

Ok so I'm looking for some thoughts on Conductive Epoxy's. I'm trying to replace a material that i can place between a QFP and the PCB to transfer heat from the component to the PCB. The material has to be able to be injected thro' a hole in the base of the PCB into the gap so something with a spray activator wouldn't work. To add to the fun the material cannot form a strong bond with either the QFP or the PCB. Anyone any idea's?.... the challenge is on!

JohnW

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Michael Parker

#7528

Thermally Conductive Epoxy | 1 August, 2001

It's amazing the eclectic applications that Silly Putty works for, what the heck, give it a shot!

One question, seriously, what are the QFP dimensions? x, y, z? can you stand for the injected material to flow past the leads?

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JohnW

#7529

Thermally Conductive Epoxy | 1 August, 2001

The material has to be injected from beneath the component, it's a 240 pin QFP, ideally I wouldn't want the material gong past teh leads as I have to have the option to rework the thing if, perish the thought, it fail's and need's fixed.....

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Michael Parker

#7533

Thermally Conductive Epoxy | 1 August, 2001

Since you are restricting the outward flow of injected material to basically mimic the package dimensions that would force you to design some sort of coffer dam beneath the part to contain the injected material.

One alternative to your quest could be Thermoelectric Modules (TEMS). I just received a monthly newsletter from empfasis that features this device. A TEM is a small solid state device that can operate as a heat pump, providing point source cooling.

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#7535

Thermally Conductive Epoxy | 1 August, 2001

Aw, we just squirt-on some chip glue. Of course it depends on the particular thermal conductivity you had in mind [but didn't say]. For instance: Loctite 3609 => 0.4 W/m�C [er sumpin like thet].

But it's real brittle. Sofer rework, ya can just pop that sucker [component to be replaced] right arf.

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Sean D

#17090

Thermally Conductive Epoxy | 3 August, 2001

Hello John,

What is the key reason requiring that this unknown material be injected from below vs. dispensed prior to placement. Is it an issue with reflow temperatures, effect on placement/thermal expansion?

Is this a true requirement or speculation based on potential complications that a dispense below the component prior to reflow could create?

Do you plan to manually dispense this material through the hole in the bottom or will this be planned as an in line process (possibly using a board inverter and a dispenser)?

Sean

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mugen

#17094

Thermally Conductive Epoxy | 3 August, 2001

We used Thermal Pads, made of black rubbery material, and sticky on both sides (like x2 side tape), intent is to place in between, BGA/QFP and the PCB board, and reduce the air-gap flow between, the PCB to SMD...

air-gap will permit air flow, that impedes the smooth reflow process, as air is a poor conductor of heat...

dunno whether this is actually what u wanna achieve?

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JohnW

#17095

Thermally Conductive Epoxy | 4 August, 2001

The reason we're putting the epoxy in at the last process is two fold really. 1. as you suggest putting the componnd down prior to placement is going to cause issues around the placement of the device. But more importantly 2. the material speced by our customer has a really low flash point in it's uncured mixed 200F(93.3ish deg C), and only about 350F after mixing / cure. Add to this that after teh material reaches 150 deg C it produces formaldehyde. the customer has specifiled that the material should be put on last, which i agree with, since if a PCB fail's (perish the thought) you don't have a major reworkign of the board. the purpose of the material is purley to act as a transfer medium to transfer heat from the component to he PCB which is acting like a heat sink.

John

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Sean D

#17113

Thermally Conductive Epoxy | 6 August, 2001

Hello Again John,

Are you injecting a like material currently? How are you doing this with your current process? Is this material being used specifically because the customer identified it as such? If this is not a current process for you, how do you plan on tooling for this in an automated format? Are you currently tooled to perform this function? How do you control the dispense of the material through a hole (no bubbles/voids/even distribution etc)?

You just seem to be creating so many potential issues with this direction. I'm just trying to get a handle on the feasability for you to find easier solutions.

Sean

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