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Thermal Pads Soldering Worry, can anybody help?

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#78458

Thermal Pads Soldering Worry, can anybody help? | 20 July, 2017

Below is an email that I copy and pasted from my customer's engineer about his concerns with the issue of the chips not reflowing (which is why I was asking about X-ray the chips to make sure they reflowed):

Can you let me know your thoughts on his concerns?

Maybe I'm worrying about something that isn't a problem, I don't know. Here is what I do know. Usually, when you connect a via to a ground plane for electrical connection, you use what is called a thermal relief. If you look at layers IN2 and IN3 with a Gerber viewer, you will see the thermal reliefs. Basically, the via is at the center of a "wheel" with "spokes" to the ground plane. This is so if the land on top of the board is real near a pad, the ground plane won't heat sink the pad during reflow.

But when you specifically want the ground plane to heat sink the pad, as is the case with power chips that have a thermal pad on the bottom of the chip, you specifically don't use thermal reliefs on the vias between the pad and the ground plane. If you took a bare board and tried to hand solder a wire onto one of these thermal pads, you would be surprised how long it would take to melt the solder on the pad because you actually have to heat up the entire ground plane to get the pad to heat up.

So when I used to use Matric to run 5 pieces of a new design, they always X-Ray inspected to make certain that the solder paste reflowed and soldered the IC power pad to the PCB power pad. The issue isn't a short, the issue is did the ground plane get hot enough to reflow the solder paste between the I.C. power pad and the PCB power pad.

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#78460

Thermal Pads Soldering Worry, can anybody help? | 21 July, 2017

Reflow oven profile and X-ray should give you all the answers that you need. Thermocouples should be attached to this ground plane.

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#78469

Thermal Pads Soldering Worry, can anybody help? | 21 July, 2017

Thanks for your reply. X-ray inspection can only tell you whether the soldering is well, but can not solve the root problem of thermal pads.

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#78476

Thermal Pads Soldering Worry, can anybody help? | 24 July, 2017

Precisely! Your reflow profile is what proves temperature on the pad is right. X-ray is your visual representation of the result - customers love to see that you are correct.

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#78481

Thermal Pads Soldering Worry, can anybody help? | 24 July, 2017

In fact, we can solve this problem easily. AOI only check whether the IC is soldered well. But we can ask our technician to make a test first to control the reflowing temperature and time.

1. using tool to test the temperature between the bottom IC and the IC pad of the PCB. Through adjusting the reflow temperature and make sure the temperature of the bottom IC reaches to about 245 degree celsius at least. 2. Making a good laser stencil, which can prevent the solder paste from flowing into the holes. 3. Reflowing time control in 80-100 seconds.

Ensuring you can handle as per these 3 steps, then you can promise the soldering is qualified for the thermal pads soldering.www.madpcb.com

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#78486

Thermal Pads Soldering Worry, can anybody help? | 25 July, 2017

There are lots of examples of changing a parameter in one direction to fix one issue causes a different problem. Once I asked a layout engineer to adjust thermal relief for through hole parts. First he said ok but a day later he said that he couldn't because of operational standards for the assembly. If you have an smt part with high thermal mass and others with low thermal mass, it can be difficult to get the peak temperature on the high thermal mass part high enough without having the low thermal mass parts above reflow too long.

The problem you talk about isn't so bad because the whole assembly gets heated up in the oven. The part does not lose quite as much heat through the thermal pad during reflow because the ground plane is also heating up from the oven.

But in any case the contradiction of needing a thermal pad for the end product operation and not wanting a thermal drain during assembly is simply a fact of life. It's why people at other companies get paid the big bucks. (it's always people at other companies getting paid the big bucks)

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#78569

Thermal Pads Soldering Worry, can anybody help? | 1 August, 2017

Thanks Dawson! it seems that it is helpful.

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#78571

Thermal Pads Soldering Worry, can anybody help? | 1 August, 2017

Really thanks for your help. I agree that this is the contradiction.

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