SparkFun Forums 

Where electronics enthusiasts find answers.

Have questions about a SparkFun product or board? This is the place to be.
By corvette123
#54711
you can use a hot air rework station also...

paste down the pads....

place the QFN.

and hold it down tight with tweezers.


they apply the hot air around the chip.

this method works well.
By NleahciM
#54816
leon_heller wrote:I hold it down whilst I tack down two opposite corners. I then drag solder all the pads with plenty of gel flux. I put a large via in the centre of the central pad, and feed solder in with a soldering iron. I use Metcal equipment. which helps a lot.

Leon
You mentioned you solder QFNs on homemade PCBs. I'm assuming you don't have a method for plating vias on homemade PCBs and the via that you're referring to is on professionally made PCBs?
By NleahciM
#54817
silic0re wrote:Hi all,

Sorry to drag up this old thread, but I have a couple quick questions! I've made footprints for two leadless QFNs (TPS63001 [3x3mm, ~12 pins], and TPS65131 [4x4mm, 24pins]), and checked printouts of the footprints with real devices to make sure they're about right. I've also made the pins extra long to have a little more room for soldering -- they extend about 1.0mm from the edge of the devices.

I'm really not sure how I'm going to solder these things, but was thinking of dabbing on some solder paste to the footprint and going the toaster reflow route. Has anyone tried this?

While I've soldered plenty of small SMD's, I don't have much experience (nor success) with leadless devices. I once tried a SCP1000 barometric pressure sensor, whose pads are actually completely under the device and unexposed to to the sides. I designed a footprint that had much longer legs (about 3mm) so that one could put some paste on the footprint then reflow it after the part was on, but this didn't have very much success at all.

I think hot air would be ideal, but I don't have the equipment -- just a normal decent soldering iron, and patience. Still, I fear a repeat of my past leadless attempt, and think the toaster oven might be a viable alternative. Any thoughts or success stories?

thanks!
Hot air works fine - but you have to remember to pre-heat the PCB. Your goal should be to get the entire PCB to just below the melting point of your solder paste, and then use the hot air to bring it the extra 5 degrees.

One method I've used in the past without any fancy equipment is to put a bit of paste on the center pad and then squish it down as much as possible, then solder the outside leads. Then, heat up the center pad from behind using a via that is placed in the pad. It'll melt the paste and get the center pad soldered.

However - if you're going to have these boards professionally stuffed - you absolutely should not do this - as it'll make it nearly impossible for your board assemblers to get a good connection to that pad, as the via will wick away solder out of the pad and into the via. Of course - there are expensive fixes for this (having the vias capped or filled) - but those add significant cost.

You can also do something similar - but use a number of vias in some sort of grid or array, and make them as small as possible. Then tent the vias with as little soldermask material as possible on the component side. Your goal would be to just barely block the hole of the via. Then you can heat up the array of vias from the backside to melt the solder paste, while also leaving the board reflowable as the tenting will keep the vias from sucking up solder.
By silic0re
#54819
I'm very happy to see all the neat discussion this has generated! hot air would of course be ideal, and I'm not sure that drag soldering would work -- I think it'd be okay for some larger leadless QFN's with big pads and lots of pad up the side, like the older Analog Devices accelerometers, but these things have two very tiny pins per millimeter with virtually no protrusion up the side.

I think I may try putting a little solder paste on each pad, melt it by just touching it with a soldering iron, then placing it onto the board and reflowing it using a toaster oven. I'm sure hot air would take care of this problem in no time, but as I'm a student I'll have to find another creative solution! :)
User avatar
By leon_heller
#54827
NleahciM wrote:
leon_heller wrote:I hold it down whilst I tack down two opposite corners. I then drag solder all the pads with plenty of gel flux. I put a large via in the centre of the central pad, and feed solder in with a soldering iron. I use Metcal equipment. which helps a lot.

Leon
You mentioned you solder QFNs on homemade PCBs. I'm assuming you don't have a method for plating vias on homemade PCBs and the via that you're referring to is on professionally made PCBs?
Yes, as a rule. I have managed it with an unplated via on a home-made PCB, though.

Leon
By NleahciM
#54888
leon_heller wrote:
NleahciM wrote:
leon_heller wrote:I hold it down whilst I tack down two opposite corners. I then drag solder all the pads with plenty of gel flux. I put a large via in the centre of the central pad, and feed solder in with a soldering iron. I use Metcal equipment. which helps a lot.

Leon
You mentioned you solder QFNs on homemade PCBs. I'm assuming you don't have a method for plating vias on homemade PCBs and the via that you're referring to is on professionally made PCBs?
Yes, as a rule. I have managed it with an unplated via on a home-made PCB, though.

Leon
Really large via + really fine tipped iron?
User avatar
By leon_heller
#54889
2mm via and largish tip Metcal cartridge. Metcals deliver a lot of heat.

Leon
User avatar
By phalanx
#54891
NleahciM wrote:Really large via + really fine tipped iron?
Here is a picture of a board I made a few years ago where I used an Allegro A3984 motor driver IC. It's a TSSOP-24 with a thermal pad on the back. I did the same thing as Leon to get solder to the thermal pad. If you take a look, I have one large via in the center of the pad. The smaller vias were to add more surface area to the ground plane connection for better heat transfer to the rear of the board. The grid around all the vias leaves that area free of soldermask.

I hand soldered the thermal pad through the large via which has a finished size (plated) of just over 1mm using a normal sized tip on a Weller iron. Unless you are doing something very specialized and tiny, there is really no need to get a very fine tipped iron. I tend to use the mid-sized chisel and screwdriver tips and have no problems soldering 0.4mm pitch devices with them.

Image

-Bill
By NleahciM
#54959
phalanx wrote:
NleahciM wrote:Really large via + really fine tipped iron?
Here is a picture of a board I made a few years ago where I used an Allegro A3984 motor driver IC. It's a TSSOP-24 with a thermal pad on the back. I did the same thing as Leon to get solder to the thermal pad. If you take a look, I have one large via in the center of the pad. The smaller vias were to add more surface area to the ground plane connection for better heat transfer to the rear of the board. The grid around all the vias leaves that area free of soldermask.

I hand soldered the thermal pad through the large via which has a finished size (plated) of just over 1mm using a normal sized tip on a Weller iron. Unless you are doing something very specialized and tiny, there is really no need to get a very fine tipped iron. I tend to use the mid-sized chisel and screwdriver tips and have no problems soldering 0.4mm pitch devices with them.

Image

-Bill
Hi Bill - the point was - he was soldering through an unplated hole. So you can't just heat up the hole itself - as it's just fiberglass. So you'd have to find a way to make contact with the pad - ideas I had were large hole + small tipped iron, or maybe filling the unplated hole with solder paste and melting that - but I'm not sure that'd work as the paste would want to cling to your iron more than melt onto the thermal slug.
User avatar
By phalanx
#54961
NleahciM wrote: Hi Bill - the point was - he was soldering through an unplated hole. So you can't just heat up the hole itself - as it's just fiberglass. So you'd have to find a way to make contact with the pad - ideas I had were large hole + small tipped iron, or maybe filling the unplated hole with solder paste and melting that - but I'm not sure that'd work as the paste would want to cling to your iron more than melt onto the thermal slug.
You won't be able to sorce enough heat from a really tiny tip to get the solder to flow reliably. If you keep your tip near the edge of the unplated (and liberally fluxed) hole and rapidly feed solder into it, you will be able to get it to flow to the thermal pad on the back of your package. If you can find some cheap parts to practice on, you will find that it isn't as difficult as you think it is.

-Bill
By static
#57063
for prototyping, i found it very handy to fabricate a small DIP module as followings:

- get a small piece of proto pcb (with grid of holes) that can contain the QFN package and the header connectors comfortably (i used x2 8pin single headers for the accelerator chip i did)
- glue the QFN chip upside down in the center
- solder the header connectors to the edges of the module board
- put some tin on the pads (not big blobs, just enough to pertrude a little)
- use very sharp tipped iron and VERY thin wire (solid core lacquered coil type of wire should do fine) to connect the pads to the points where headers were soldered to the pcb.

and bingo - you have a qfn -> dip module of your own making. I'll take some pics of mine if i remember.
It is important to use a very thin wire for this, because if it's too thick, putting stress on the wires while handling/soldering them may lift/ruin the pads on the chip (no joke, happened to me)
By funnynypd
#69163
Also remember using some non-clean type of solder paste or flux, clean the residual out under those chips are the pain of the ***.

We made some mistake by using water-soluable flux, and we have to do it all over again because the residual cannot be cleaned, overtime, it will start to make issues on the circuits. Luckly we found it right after the boards are assemblied and all boards got fixed immediately.