[Adam S]

Normally any QFN packages and the like that need to dissipate heat will
have a thermal pad designed to transfer heat from die to a copper ground
plane via thermal vias. Is it possible to improve the thermal
conductivity by adding a aluminum heatsink directly on the PCB surface
opposite the QFN package ?

 

[John Larkin]

Normally any QFN packages and the like that need to dissipate heat will
have a thermal pad designed to transfer heat from die to a copper ground
plane via thermal vias. Is it possible to improve the thermal
conductivity by adding a aluminum heatsink directly on the PCB surface
opposite the QFN package ?

Yes, we do that. The important thing then becomes to establish a good
heat flow path from the thermal pad to the sink. That is typically a
copper pad on the top side that solders to the thermal pad on the ic;
a number of thermal vias; and a biggish pad on the bottomside that you
bond the heatsink to somehow. The vias may then be a major source of
thermal resistance, since they will probably be few and small.

An inner-layer heat spreader, a copper pour, can help, usually the
ground plane itself. Make that layer 2 if possible and nail it to the
heatsink pour with additional, largish vias outside the footprint of
the ic.

http://s2.supload.com/free/AmpBoard.JPG/view/
http://s2.supload.com/free/AmpBottom.jpg/view/

John

 

[Adam S]

Yes, we do that. The important thing then becomes to establish a good
heat flow path from the thermal pad to the sink. That is typically a
copper pad on the top side that solders to the thermal pad on the ic;
a number of thermal vias; and a biggish pad on the bottomside that you
bond the heatsink to somehow. The vias may then be a major source of
thermal resistance, since they will probably be few and small.

An inner-layer heat spreader, a copper pour, can help, usually the
ground plane itself. Make that layer 2 if possible and nail it to the
heatsink pour with additional, largish vias outside the footprint of
the ic.

http://s2.supload.com/free/AmpBoard.JPG/view/
http://s2.supload.com/free/AmpBottom.jpg/view/

John

Thanks for the photos. thats some densely packed PCB !
I am using 2 layer board so unfortunately I cannot take advantage of the
extra copper you mentioned above. What have you used to bond the
aluminum heatsink to the solder pad, I'm assuming the pad is not covered
in soldermask ?

I was thinking epoxy. I would assume that the epoxy film thickness after
pressing it down firmly would reduce to less than a few 10s of microns.

Will a grid of large 1mm diameter vias improve thermal conductivity
between the IC's thermal pad and the copper pad on the opposite side of
the PCB ?

 

[[email protected]]

Thanks for the photos. thats some densely packed PCB !
I am using 2 layer board so unfortunately I cannot take advantage of the
extra copper you mentioned above. What have you used to bond the
aluminum heatsink to the solder pad, I'm assuming the pad is not covered
in soldermask ?

I was thinking epoxy. I would assume that the epoxy film thickness after
pressing it down firmly would reduce to less than a few 10s of microns.

Will a grid of large 1mm diameter vias improve thermal conductivity
between the IC's thermal pad and the copper pad on the opposite side of
the PCB ?

Are you familiar with the on-chip diode method to measure die
temperaure? If not, I'll write it up. Obviously, this is to be used to
figure out the effectiveness of whatever scheme(s) you come up with.
This is probably a situation where practical measurements are better
then theory.

Some heat leaves the chip via bond wires, so extra copper on ground or
supply also helps to cool the chip.

 

[John Devereux]

[email protected] writes:

[...]

Are you familiar with the on-chip diode method to measure die
temperaure? If not, I'll write it up. Obviously, this is to be used to
figure out the effectiveness of whatever scheme(s) you come up with.
This is probably a situation where practical measurements are better
then theory.

That sounds interesting - I for one would like to hear more about it.
Where do you get the diode? Forward bias the ESD protection?

 

[Adam S]

Are you familiar with the on-chip diode method to measure die
temperaure? If not, I'll write it up.

Yes, I'd interested. Assuming I find a diode , it'll need to be measured
during power up. As John Devereux pointed out, a forward biased ESD
protection diode on one of the logic inputs may be my only option.

Is it necessary to do a two point temperature calibration and
extrapolate Vd vs T with the diode equation ?

 

[[email protected]]

Yes, I'd interested. Assuming I find a diode , it'll need to be measured
during power up. As John Devereux pointed out, a forward biased ESD
protection diode on one of the logic inputs may be my only option.

Is it necessary to do a two point temperature calibration and
extrapolate Vd vs T with the diode equation ?

Yes to the forward biased protection diode. Rather than using a diode
equation, just feed the diode with a current source, which can be as
simple as a bench supply and resistor. Then calibrate the diode by
putting the part in an oven, sweeping the temperature. This realy only
has to be done from room temp up to the maximum temp you want for the
die. I'm not sure I'd trust a diode equation since the ESD diode often
has some resistance in the model. However, I never tried to curve fit
the data.

The current should be low, say 10uA. The DVM should be good, i.e. high
impedance and at least 5 digits. Note you are injecting carriers into
the chip, so look for odd behavior. You might also discover a factory
test mode. ;-)

You can use this scheme to test thermal shutdown in regulators.The
part will oscillate, i.e. you can see the hysteresis.

 

[John Larkin]

Thanks for the photos. thats some densely packed PCB !
I am using 2 layer board so unfortunately I cannot take advantage of the
extra copper you mentioned above. What have you used to bond the
aluminum heatsink to the solder pad, I'm assuming the pad is not covered
in soldermask ?

Silicone grease, and no solder mask. But the mask wouldn't add a lot
of theta, I'm guessing. We use grease so we can remove/rework the ICs
if necessary, and that involves removing the sink and heating the
board. These chips are leadless Hittite 20 GHz distributed amplifiers,
$200 each.

I was thinking epoxy. I would assume that the epoxy film thickness after
pressing it down firmly would reduce to less than a few 10s of microns.

The limit on thickness, with epoxy or grease, is probably the flatness
of the heatsink-pcb interface. Thermal epoxy or filled silicone grease
will squash down below 100 microinches, 2.5 microns, which is about
all I can reliably measure.

Will a grid of large 1mm diameter vias improve thermal conductivity
between the IC's thermal pad and the copper pad on the opposite side of
the PCB ?

Absolutely. The vias are the only decent heat path from the ic's
thermal pad to the world. Even better if they're filled with solder,
although that will tend to make bumps on the opposite side.

John

 

[Robert Latest]

http://s2.supload.com/free/AmpBoard.JPG/view/
http://s2.supload.com/free/AmpBottom.jpg/view/

Why the concentric arrangement of SMB sockets and associated parts? I like
it.

robert

 

[John Larkin]

Why the concentric arrangement of SMB sockets and associated parts? I like
it.

It's mixing (summing) ten signals coming in from other boards, over
coax cables. The radial arrangement keeps the mixer paths symmetric
(bandwidth here is about 3 GHz) and we liked the way it looks!

The connectors are MCX's, which look a lot like SMBs.

This is the final stage of a laser waveform modulator for the NIF
laser. There are 48 modulator boxes, one per "quad" of four lasers,
total 192 beam lines delivering about 1.5 megajoules of UV to the
target chamber.

John

 

[[email protected]]

Yes, I'd interested. Assuming I find a diode , it'll need to be measured
during power up. As John Devereux pointed out, a forward biased ESD
protection diode on one of the logic inputs may be my only option.

Is it necessary to do a two point temperature calibration and
extrapolate Vd vs T with the diode equation ?

I'd like to know what you learned from this experiment.

Years ago, one of the apps engineers at Maxim took a chip that
depended on power supply copper to cool it. He put in on a large PCB
and kept cutting back the amount of attached copper with a dremel.,
yielding theta JA versus attached copper. I don't think it ever left
the company. Yeah, this isn't the same as the internal copper slug,
but a similar experiment might be useful.

To really get good thermal flow from the pins, the chip need fat bond
wire. [Flow proportional to the cross sectional area of the wire, so a
little thicker wire is a big deal.] Generally the factory doesn't
provide that kind of information.