TQFP soldering troubles - Page 1
Forum section: electronics / projects

author: Rachie5272
author_rank: Regular Contributor
timestamp: March 06, 2020, 11:16:47 pm
content: I've been having to rework a bunch of 0.5mm pitch TQFP chips lately, and for some reason they've been very difficult. Harder than QFN. The problem is solder bridging. Everything is swimming in really nice BGA rework flux gel, and I can very easily drag excess solder to one end of pins, but then what? The leads form a nice capillary channel, which preferentially holds the solder over the iron tip.I've tried sever tips: J hook, well, chisel, and even a big drag soldering tip. Solder wick is the only thing which works, but then I'm left with too little solder and mechanically weak joints.Any suggestions? I'm already switching to QFN for the next design.

author: Wilksey
author_rank: Super Contributor
timestamp: March 06, 2020, 11:24:36 pm
content: I use solder wick to remove excess solder, do you have any? Or use less solder to begin with, use thinner solder.

author: KL27x
author_rank: Super Contributor
timestamp: March 06, 2020, 11:51:03 pm
content: Either a tinned face-only bevel/hoof or a regular bevel/hoof. But it has to be 2.5mm plus or minus half a mm.With a regular hoof, carefully wipe the entire tip and only apply solder to the cut face.Don't hold the tip completely parallel with the pcb, because that will squish your solder reservoir. Tilt the tip to create clearance for this solder bead that you carry on the bottom cut face of this hoof tip.Try running the edge of the bevel just along the toe of the pins. Or you can try laying it on top of the pins.If you get a huge glob on the first few pins, just stop and do the other side, then finish this one when your iron is running lower on ammo.The TFO is the best at sucking out these big shoulder bridges. But only Hakko seems to make them. The regular bevel will do near the same thing, esp after months of use like this and the sides start of the tip start to oxidize a little.

author: james_s
author_rank: Super Contributor
timestamp: March 06, 2020, 11:58:41 pm
content: I use a regular 1/8" wide chisel tip.Try using less solder. Rather than dragging, I tend to melt a bit of solder onto the tip of the iron and then touch it to the tip of a few pins where they meet the pads and allow the solder to wick up under the pins. Occasionally I get a bridge at which point I either wipe the tip of the iron and use it to pick up the excess solder or use a bit of wick. It takes practice but it's not too hard.

author: KL27x
author_rank: Super Contributor
timestamp: March 07, 2020, 12:00:49 am
content: The sharpness of the edge makes a big difference, too. E.g., the two "spoon/well" tips I have, I don't use, ever. The edges are way too rounded, and the utility of these tips is very limited for me.The hoof with a sharp edge is also better at doing the toe drag and for QFN's, you need a really crisp edge. This crisp edge also helps with solderwicking and keeping the bead on the face of the tip, rather than wrapping up the heel or side.This crisp edge is how my hoof tips die. When the edge gets too rounded to the point you can't get it back without wearing through to the copper, then it's done.Also, the angle of the cut face matters. You want it to be rounder. Something more closer to 45 than 60.

author: Rachie5272
author_rank: Regular Contributor
timestamp: March 07, 2020, 05:45:58 am
content: Thanks everyone. The hoof tip worked well. Unfortunately, my board still doesn't work. Back to debugging...

author: angrybird
author_rank: Regular Contributor
timestamp: March 07, 2020, 05:50:37 am
content: You can also get a flux pen, I use kester 959T, put it across the leads then reflow it with the tip while smoothly and quickly drawing it away from the IC. Between the surface tension of the solder with flesh flux and the small amount removed from the tip, you can sometimes clean up most of the IC this way after drag soldering it.

author: Shock
author_rank: Super Contributor
timestamp: March 07, 2020, 07:12:59 am
content: This video is a good one to watch. You don't need tons of flux as your work time should be limited.

author: Rachie5272
author_rank: Regular Contributor
timestamp: March 08, 2020, 05:56:22 pm
content: Does anyone know of a source recommending which tips to use in what situations? I've never thought to use the hoof tip until now.

author: Shock
author_rank: Super Contributor
timestamp: March 08, 2020, 06:56:16 pm
content: Something like this:Bevel, hoof, wave, gullwing style tip for drag solderingChisels in various sizes for through hole plus chip smdKnife for rework and through hole drag solderingHook for rework, touch ups and difficult accessBlade and large chisels for pad cleaning, heating large areas and tinning wirePointy tip to poke in holesFor most general soldering I think the chisel makes the best contact to the pad and leg of the component. It works really well doing the presolder and reflow method of smd chip component soldering. Downside is you need a selection of different sizes.The knife does a bit of everything. Hooks are a bit dual purpose as well, if the bend is tinned it acts like a large chisel, it's usefulness depends on it's shape and tip.

author: KL27x
author_rank: Super Contributor
timestamp: March 09, 2020, 12:45:56 am
content: My algorithm is based on results with SMD PCB's and SMD and thru hole prototyping.I have about 15 tips. 13 of them are bevel tips. Then I have a knife and a pointy BR tip.I use a 2.5mm TFO hoof tip for almost everything.1mm - 2mm TFO hoof tip for point to point bodge wiring/prototyping.Knife tip is occasionally used for removing passives in a batch rework. When you do production soldering, you don't mess up one board, you mess up dozens by the time you notice. The knife can remove passives without adding extra solder, you just leave the passive on the side of the knife and it slides up when you get the next one, until you have a spatula with 6-12 passives stacked on the knife face. With the bevel you have to get a bead going, and then you have to do something with the bead and the little passive swimming in the bead.I have never used the BR tip other than messing around. It looks cool, though.

author: SiliconWizard
author_rank: Super Contributor
timestamp: March 09, 2020, 01:29:59 am
content: I often use a chisel tip (thin, but largish) for this and don't do drag soldering - I solder several pins at once (according to the width of the tip), then the next pins, etc. Doesn't take that much more time than drag soldering when you get the hang of it, and it doesn't make hardly any bridge. Also, I use gel flux. More effective here than liquid flux.Sure for the very occasional bridge, you use fine solder wick (don't use a too large one that will tend to suck all solder in) and just for less than a second.

author: Shock
author_rank: Super Contributor
timestamp: March 09, 2020, 01:57:36 am
content: I mentioned this the other day, if you are soldering efficiently you shouldn't need to buy extra work time by adding excessive flux.In that video I posted John Gammell said to not use wick to fix smd drag soldering bridges. I think it makes sense to add a little flux if required and just use a clean tip to fix it. If it was any worse it's a technique problem and that should be remedied first.

author: KL27x
author_rank: Super Contributor
timestamp: March 09, 2020, 02:29:12 am
content: The only major problem using "too much" flux is the fumes and the mess when it runs through the holes or the routered edges of the board, making the board stick to w/e is under it. I usually just put the piece of cardboard under the board to soak up a bit of the flux.I have begun to use a brush to get just the right amount when prepping boards for production soldering. But most of the time, too much is mainly theoretical.QuoteThis works ok. But its ergonomically bad. A knife tip is better at this due to the angles. Aside from the ergos demanding just the right angle, this dumps a lot of heat into the pcb (and if you use an old pre-cartridge iron, this can mean a lot of heat into the handpiece.) This tip is also not as good at sucking out bridges, so it's not as forgiving of user error.I find it actually works a little smoother if you angle the edge of the knife so it's almost flush with the row of pins, but slightly angles so only the tip actually touches the pins.The beauty of the hoof is multiple.1. The cut face is nearly horizontal and on the bottom; this allows it to suck solder bridges out without working very hard against gravity. But with a bead on there and a bit of tilt, it allows the tip to feed the joint very effectively. It's just the right balance.2. The curved sharp edge is what rolls against the side of a QFN or the toes of a row of pins. This makes for a small contact area of the edge of a large thermal mass to slide over the pins, contacting just a couple at a time. And the curve allows the solder from the cut-face-bead to flowing around the contact point into the joint.3. The cut-face/solder-reservoir is an oval. Surface tension of solder makes it want to bead into a sphere. On a knife tip, this means the solder beads wants to draw away from the pointy end. On a chisel... it doesn't matter cuz the entire tip and shaft is tinned, and the bead is just an uncontrollable blob, anyhow. When the solder is on the oval cut-face on the bottom of a bevel, it will reach the entire edge, all the way to the edge. This round surface is also efficient regarding surface area:volume of the tinnable area + bead, so the tin doesn't dry out as fast between joints of while on the stand, or make as much smoke.The major downside of a TFO hoof is feeding solder onto the tip. You want to feed solder on the cut face, so this is a bit of twisting and turning if you use "thru hole" methodology. For SMD work, a 2.5-3mm hoof can solder multiple components without stopping, and I just rig my solderwire to hang at the edge of my FOV, for reloads.When you have a bunch of SMD assembly to do, but not quite enough where you fancy putting together and paying for a PCBA order, remember this. And maybe buy some tips to play around with for the job. The chisel tip has no place in SMD soldering, any more than any tip can be used to solder anything, if you really don't have that much to solder in the first place.;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;Another thing: With the hoof tip, I routinely do both rows of a gullwing without flipping the board around. And I will do up to 2 sides of a QFN. The front and the right side. Even with a knife, I can't do that, because the angle has to be so exact. It's hard to find. Forget the perpendicular chisel doing anything like that; you probably have to come in somewhere from the east, and have the row of pins lined up just right with your body. With the large bevel, the right angles are a wide range which is extremely forgiving.In this case, with the DIP doing both rows, it's just a 6-7 degree twist of the iron CCW and I don't even need to move or replant my hand. I use the front right edge of the bevel for the front row of gullwing pins (usually top of the toes, but also can be done on the toes). Then I use the left heel of the bevel for the top row; here you can do either, as well.With the QFN doing the second row is just a 1-2 degree twist of the iron and blam, done. Front right of the bevel for the front row. CCW twist of 1-2 degrees, then top to bottom of the right side of the chip, using the front left part of the bevel. (The bottom cut-face of the hoof is always tilting up on the side that is opposite from the row of pins, in general, when drag soldering. More tilt = more stingy; so if you just loaded up on a big blob solder, you can use a bit more tilt. Less tilt will "squeeze" the solder bead against the board, force-feeding the joint, and you can bridge your gullwing IC's.)

author: SiliconWizard
author_rank: Super Contributor
timestamp: March 09, 2020, 02:28:36 pm
content: Quote from: KL27x on March 09, 2020, 02:29:12 amI guess it all depends on the iron you're using and the way you hold it. I mostly use a JBC T210 for small SMD jobs these days, and can hold it in a way that it's not incomfortable at all to do this. But hey, I admit I only do a few prototypes a year and would likely use other methods if I had to do a lot more.I actually started with this method when I had to solder fine-pitch Hirose connectors years back. I didn't really find any other usable method back then, as dragging a tip too close to the plastic part would quickly melt it. Then I found the method rather convenient, and built a habit of using it for pretty much any fine-pitch soldering jobs for similar pins.As to transfering heat into the PCB - not really. Not the way I do it at least.Now it's all in picking the "right" tip for the job. Not all chisel tips will work well with this method. That's again where I appreciate the available JBC tips.Regarding "hoof" tips - I think they are more commonly called "bevel"? - I have one of them. I don't use it very often, but I'll give it a chance for QFPs and see how that goes.

author: KL27x
author_rank: Super Contributor
timestamp: March 09, 2020, 11:07:55 pm
content: QuoteThe chisel is a 90 degree perpendicular edge. So it doesn't matter how you hold it. It will only form a straight line that is 90 degrees away from where you plant your right hand by the time you orient this edge flat to the PCB. This is main reason why it's an ergomonic fail right out of the gate. Then to drag this edge across a row of pins, you have to move the entire iron sideways. This is ergonomic fail number two.The 45 degree slant of a knife tip, boom, automatically makes it a million times better for almost anything SMD. There are other differences, though. The way Hakko makes them, anyway:1. the knife is tinned-face-only. Only the faces of the edge bevels are tinnable. The rest is chrome plated.On the chisel or screwdriver tip, the entire shaft end of the tip is iron. The knife tip is better at sucking bridges and controlling solder due to this.2. The knife tip comes to a true edge. The chisel is kind blunted over. For QFN's, this lack of a sharp edge makes the chisel unable to solder a QFN until you get very small chisels that are basically a conical that thinks it is something special but it's not.QuotePlease refer to my notes about bevel tips. There is variance between manufacturers. Hakko's bevel tips are sublime, for instance. Some of the clones and the high quality Plato aftermarket spoon versions do not have the proper features of a "hakko" bevel tip, having cut angles that are too slanted and/or rounded edges. Sharp edges are very important for bridge sucking. The size is also critical. When solder beads, it reaches a certain size of ball before gravity collapses this shape. 4mm is too big; the bead will collapse and be uncontrollable when it gets too large, and when a little solder is on there, it won't want to spread across the entire face. Too small, and you have to reload too often. 2.5-3mm with a 45 degree cut face is ideal with lead solder, IME. As you go smaller, it will work with slantier cuts angles.

author: jmelson
author_rank: Super Contributor
timestamp: March 10, 2020, 07:52:32 pm
content: First, you need good alignment between part and pads. The narrower the pads are, the less likely to get bridges. Are you using paste or wire solder? With paste, lay a VERY thin strip down the center of each row of pads before placing the chip. With wire, use the finest wire you can get. I have some .010" (0.25mm) solder that works the best. Clean the pads with braid before getting ready to place the part, then put a dab of solder on two corner pads. Align the chip and solder those two pads. You can reheat the pads and adjust the chip for most accurate alignment. Then, just apply the LEAST possible amount of solder and do 2 - 3 pins before applying more solder. In some cases, the amount of solder plated on the PC board is enough to make good joints, all you need to do is supply the flux.Finally, if you don't have a stereo zoom microscope, you need this BADLY to do 0.5 mm lead pitch.Jon