Voltage Regulator...Reflowing the large footprint D²PAK - Page 1
Forum section: electronics / general

author: Falcon69
author_rank: Super Contributor
timestamp: October 26, 2014, 05:28:28 am
content: Okay,So I'm busy trying to layout this circuit on a pcb to have them printed up.This particular voltage regulator I am working with is a D²PAK and has a very large pad for the OUT. Smaller pads for the IN and ADJ. My question is, will this very large pad reflow okay, or are there steps I need to take to help it reflow? Do I need to place vias in the pad to help? Or does that make it worse? Do I need to split the pad into section under the chip? I just don't want any problems with the reflowing.http://www.st.com/web/en/resource/technical/document/datasheet/CD00001883.pdfThe other thing I'm confused about is....this regulator has the OUT on the large pad, where other voltage regulators I've seen, the big pad is for ground. Why is this chip different?

author: zapta
author_rank: Super Contributor
timestamp: October 26, 2014, 06:05:53 am
content: I used something similar here (PDF), digikey LT1764AEQ#TRPBFCT-ND and reflowing was normal with no special issues.https://github.com/zapta/power-monitors/blob/master/pmon_3v8/eagle/pmon_3v8_board.pdf?raw=trueThe vias are to connect the large pad to the ground plane on the bottom layer (electrical and thermal). I had two stencils, first a mylar one with one big opening for the entire big tab and a second, metal, where the vendor (Electro) made the big opening into a grid, reducing the overall opening to ~80%. In both cases reflowing was straight forward.Hope it helps.

author: Falcon69
author_rank: Super Contributor
timestamp: October 26, 2014, 06:13:29 am
content: Ya, the only thing is, my chip's big pad is for the output (positive to circuit). Not the ground as in yours. But thanks. Did you change the ramp or soak or anything to get it to reflow? or it just did it?

author: IconicPCB
author_rank: Super Contributor
timestamp: October 26, 2014, 08:03:39 am
content: Big pads and accordingly large electrodes can be a problem at times.Pay attention to the stencil opening, reduce the amount of solder otherwise the component might float if too much paste applied.Pay attention to the copper thermal relief around the pad to minimise heat-sinking effect of copper pour around the pad.Otherwise no major drama.

author: Falcon69
author_rank: Super Contributor
timestamp: October 26, 2014, 08:24:26 am
content: okay, how much should i reduce the stencil by? I believe I can do that in DipTrace.

author: ludzinc
author_rank: Unknown
timestamp: October 26, 2014, 08:31:49 am
content: Just the one regulator?Hand solder?If not datasheet usually defines required solder paste sizing.

author: Falcon69
author_rank: Super Contributor
timestamp: October 26, 2014, 08:44:56 am
content: Yes, Data sheets define that. And I have drawn the correct size footprint, according to the data sheet, however....I am just a hobbyist with a home made toaster oven reflow oven. I have more than 1 of these to make. I would rather NOT hand solder them.I am not sure about making a grid type pattern for the solder paste stencil as zapta has said his pcb manufacturer did. I know from experience when dealing with the heatsink fan onto a computer CPU, that if any air is between the heatsink and the CPU, the chip heats up very fast. So, I want to make sure I get the proper amount of solder underneath, without an air gap anywhere. But, I also understand that too much solder paste, makes the chip float and the other pins may not connect. So I need to figure a way to find a balance.

author: zapta
author_rank: Super Contributor
timestamp: October 27, 2014, 03:20:33 am
content: Quote from: Falcon69 on October 26, 2014, 06:13:29 amDidn't have to change anything, using same profile #2 of the T962 oven. I am posting now from a phone but I think I posted a picture of the grid and asked if it's normal somewhere in the forms. My stencil vendor is Elecrow, they often fine tune my stencils, no problem so far.

author: Falcon69
author_rank: Super Contributor
timestamp: October 27, 2014, 05:56:48 am
content: Okay, and Elecrow did it automatically without you asking them? I may try Elecrow next run. I just don't want someone completely changing the design. The last company I used changed the copper pour on me and I received boards where the copper pour wrapped around holes I did not want it too.