solder paste volume on a qfn with lengthened pads - Page 1
Forum section: electronics / general

author: sheitbrink
author_rank: Newbie
timestamp: March 05, 2018, 05:50:01 pm
content: I have a question about QFN stencil design. I would I have a QFN part that I would like to be able to put on using a paste stencil, but have the pads lengthened enough that I can get a soldering iron on the pads to fix any bridging/cold joints if they arise. I was wondering, for one, if this was a good idea. And also, does anyone know of any formula to calculate the paste volume you need to get a good solder joint?My best guess right now is to split the difference between the original pad dimension, and the new extended pad dimension.I’m probably just going to order some stencils from OSH stencils. So we are talking about a 4mill stainless steel stencil with no fancy electro polishing or variable stencil width (aka stepping).

author: DerekG
author_rank: Frequent Contributor
timestamp: March 06, 2018, 12:05:07 am
content: Quote from: sheitbrink on March 05, 2018, 05:50:01 pmExcellent idea if you have the room on your board. Best to change the component in your library. Simply make the "X" (or it might be the "Y") pad dimension larger.QuoteCheck out the manufacturers data sheet & do not go below the pad size they recommend.QuoteThat is pretty thick. I suspect it is thicker than the manufacturer recommends. If it is too thick you will get solder bridges when when putting the board through the vapour phase (or infrared) heating oven.If you are going to solder this by hand with a soldering iron, I would recommending using regular hard solder & not use solder paste at all. Solder the opposite corners of the component first to lock it into place. There are a few uTube videos on how to to this with hard solder. Make sure you have some good solder flux on hand.

author: sheitbrink
author_rank: Newbie
timestamp: March 07, 2018, 05:13:16 pm
content: Thanks for the input.The big reason I don't start out by hand soldering is because these parts all seem to have ground pads on the bottom that absolutely need to be soldered. It would be really nice if someone came up with some equations for a good paste volume to start out with. It seems like it should be something that should be in some IPC document somewhere. I suppose for prototypes I could open up a large hole in the bottom, under the bottom pad and do the wire solder feed up trick.

author: cstratton
author_rank: Regular Contributor
timestamp: March 16, 2018, 04:13:17 am
content: Especially if you are going to be watching these flow (with an air tool, skillet, or small oven) I'd probably not worry so match about the solder aperture for the edge pads - even making it full size will probably work, since the solder will need to wet the larger area of the extended pad.Instead, where more of your attention may actually need to go is on getting the correct amount of solder on the central ground or thermal pad. Not enough and you have only a partial connection area. But too much, and the chip will float on the central puddle creating a gap under the edge contacts, which the solder may not reliable fill, meaning you end up with missing connections.The situation with leadless parts like QFN's is quite a bit different than with fine pitch leaded ones like TQFP's - on a TQFP you are more likely to bridge than have a pin not connected. But on a QFN you almost need sloppy hand syringing to get a bridge, while the smallest mistuning of process can yield an open, often only a hairline gap that you have to look at under strong magnification at just the right tilted angle to see.

author: ar__systems
author_rank: Frequent Contributor
timestamp: March 16, 2018, 01:11:03 pm
content: It is a good idea. Don't worry about bridging. In my experience QFN is almost impossible to bridge even on purpose, unless you make exposed pads over 2mm long.On one of my boards I needed to fix a PCB error by connecting two adjacent pins of a 0.5mm QFN. No way... I had to use pieces of wires to make the connections.

author: ar__systems
author_rank: Frequent Contributor
timestamp: March 16, 2018, 01:15:35 pm
content: Quote from: cstratton on March 16, 2018, 04:13:17 amThat's an unlikely scenario. Surface tension will pull the part down to make full connection. This is actually helpful to ensure all of the pins are connected.

author: nisma
author_rank: Regular Contributor
timestamp: March 16, 2018, 05:32:24 pm
content: On literature, the reduction for solder paste on QFN is between 80% and 50% . Basically it depend more on vias inside center pad instead of thelongened pins what ratio is used. Then use the same ratio for the pin, reducing 0,15mm from pin and then the length depends on the ratio used.

author: asmi
author_rank: Super Contributor
timestamp: March 17, 2018, 06:24:00 am
content: I've had some shorts between adjacent pins on a QFN, but never had an open connection. These shorts are usually very easy to fix - just apply a ton of flux and touch it with an iron. This is often all it takes to fix the problem. If that doesn't work, you will need to remove excess solder with a wick or a solder pump.As for footprints, I always follow chip manufacturer's recommendations if they exist, otherwise just generate IPC-compliant one. They usually have pads which extend a bit from under the package, so touch up is easy. I use reflow oven for soldering as it's the only reliable way to avoid uneven heating, and more often than not e-pads are connected to massive ground planes which are rather tricky to heat up without oven, as removing the heat from the package is the very reason they exist.

author: tautech
author_rank: Super Contributor
timestamp: March 17, 2018, 06:51:02 am
content: Watch this for another take on how to use paste:https://www.eevblog.com/forum/beginners/so-surface-mount-it-is/msg1442590/#msg1442590And another a couple posts later:https://www.eevblog.com/forum/beginners/so-surface-mount-it-is/msg1442691/#msg1442691Not saying it's right or wrong but for a SMD novice I thought it was a new and novel approach.