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JEDEC MO-240 AA ?

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Matthew Lamkin View Drop Down
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    Posted: 23 Jan 2015 at 5:30am
HI, for the JEDEC MO-240 Var AA for a Fairchild FDMS7650 etc.

This is like an SO8 with a big heat pad inthe middle where it is connected to 4 of the outer pads.
In this part it has 4 common pads for the "Drain".


If the component layout for this has the big centre pad and the 4 drain pads all within the same sodler resist clearance, so that there is no sodler resist between the 4 drain pads and the heat slug - when reflow soldering would there be a tendency for the solder from the 4 pads to wick back onto the heat slug leaving them with poor connection?

The manufacturers recommended footprint is:


Now given that they are bonded to the same heat slug on the component I'm not sure it really matters.
In the past I usually make this with a big rectangular pad for the slug with its own solder resist and 4 separate pads for the drain connections. This gives each one their own solder resist shape & hence a gap between the 4 & the slug.

So I am wondering whatIPC7351 says about it? what others would do about the above and where to start in the wizard for this - what SMT family to choose to start etc?

Cheers,
Matthew

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jameshead View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote jameshead Quote  Post ReplyReply Direct Link To This Post Posted: 23 Jan 2015 at 6:13am
We use a similar packaged device, Texas Instruments CSD16340Q3.  We use the manufacturer's recommended land pattern but created on PCB Libraries.
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Tom H View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Tom H Quote  Post ReplyReply Direct Link To This Post Posted: 23 Jan 2015 at 8:25am
IPC does not have any guidance on non-standard component packages.

You should use the mfr. recommended pattern. This is a very easy part to create in FP Designer.

In the Sample FPX that comes with Library Expert, look at these footprints -

  1. FAIRCHILD_FDMC8878

  2. IRF_IRFHM4226TRPBF

  3. FAIRCHILD_FDMS86150

  4. VISHAY_SI7115DN


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Matthew Lamkin View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Matthew Lamkin Quote  Post ReplyReply Direct Link To This Post Posted: 23 Jan 2015 at 8:29am
What about my thoughts on the solder resist between the smaller & large pad?

When creating the component in FPX - what SMT family should I start with?
P.S> I only have lite at the moment, the full version complained of a license problem that I have not got around to sorting yet.

Cheers,
Matthew

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Tom H View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Tom H Quote  Post ReplyReply Direct Link To This Post Posted: 23 Jan 2015 at 8:41am
Nick will help you with your license issue.

The past mask of the Thermal Pad should not touch the paste mask of the smaller pads.

We can relocate the Thermal Pad Paste Mask internally and then output a XML FPX file that can be opened in Library Expert. Do not co-minge XML FPX with ASCII FPX as they are 2 different formats.

I would highly recommend to use Parts on Demand and Request a Part - www.pcblibraries.com/pod

Here is a XML FPX sample of the Thermal Pad Paste Mask moved away from the smaller pads -

Sample_Thermal_Pad_Paste_Mask_XML_FPX.zip

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Matthew Lamkin View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Matthew Lamkin Quote  Post ReplyReply Direct Link To This Post Posted: 24 Jan 2015 at 9:13am
I'm away this week so will not be able to sort out any license problems.

Tom, Solder resist is not solder paste :)

Bit IMO the same should apply to the resist as the paste, that way the thermal pad paste does not get anywhere the outer pads irrespective of whatever the copper layout it.
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Tom H View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Tom H Quote  Post ReplyReply Direct Link To This Post Posted: 24 Jan 2015 at 10:07am

The Solder Mask is removed from all metal surfaces.

Full Paste Mask coverage is applied to the Lead Terminals and 50% reduction on the Thermal Pad.

There doesn't seem to be any guidance from the component mfr. (CM) on this subject of "Solder Dam" that would prevent the Lead Terminal solder from wandering onto the Thermal Pad during reflow. I would think that the Solder Mask would have a break line to prevent that. The CM's should have better drawings and illustrations.

I'll bring this subject up at the next IPC-7351 executive committee meeting at IPC APEX on Wednesday February 25. But I need to gather information from the industry and assembly shops to present it to the committee.


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Post Options Post Options   Thanks (0) Thanks(0)   Quote Matthew Lamkin Quote  Post ReplyReply Direct Link To This Post Posted: 25 Jan 2015 at 3:24pm
Cheers Tom.

It probably won't actually make any difference as the 4 drain pins are connected to the tab but IMO it will lead to poor looking joints on those pins which no one likes to see. :)
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