Choosing solder paste and needles

[neslekkim]

I just got some PTFE/Teflon lined needles today. What a HUGE difference! I was able to get the paste through a tiny 25ga needle and successfully hand solder a .4mm QFN without the usual bridges. That is a big upgrade for me. Love these things.

Does it exists any guide for sizes of the needles that are usable?, or do you typically use same size for most of the jobs?

[Ysjoelfir]

Video of dispenser and air compressor

Hi coflynn,
I have one of those units you showed in your Video lying around for a few years. My father got it from the trash of a dentists workshop that they renovated and I was about to put it back into the trash soon as I didn't know what it was and how to use it, so it was flying around from one corner of the shop to another. Mine is genuine build in germany, but looks totaly identical (except from a broken air connector). Could you send me a instruction manual or tell me how to use that? AND show some pictures of the syringe connector?
Thanks!

[plazma]

I use a very thin and a quite short needle on my paste syringe. The needle have been lubricated with a special lubricant. I have to check the brand and code at work.
Without the lubricant it would be impossible to use such a thin needle. Without such a thin needle it would be very hard to manually apply paste to small bga pads.

With a pneumatic dispenser and the lubricated small needle it's surprisingly fast to add paste to a 100+ pad bga footprint.

Finally remembered to take a photo of my syringes.
I use Nordson ValvePurge for lubing the needle. It makes it possible to use really thin needles even though lubing is not the intended use of ValvePurge.
The top most syringes are two different brand fluxes I use.
The solder paste syringe got a thin enough needle to paste BGA pads manually.
The bottom syringe got some heat activated epoxy. I use it to glue components into place (mostly bottom BGA chips of a package-on-package combo when changin the top RAM chip).

[rx8pilot]

I just got some PTFE/Teflon lined needles today. What a HUGE difference! I was able to get the paste through a tiny 25ga needle and successfully hand solder a .4mm QFN without the usual bridges. That is a big upgrade for me. Love these things.

Does it exists any guide for sizes of the needles that are usable?, or do you typically use same size for most of the jobs?

I believe there is a whole range of sizes available. I only use the PTFE needles to the fine pitch work so the 25ga is all I have. For larger pads, the steel needles flow just fine so I don't bother with the more expensive and fragile teflon ones.

They have been so helpful, really saving some time when I needed it most.

[KL27x]

tiny 25ga needle and successfully hand solder a .4mm QFN without the usual bridges.

Hand solder? Hmm, wonder if you could do that easier with the right iron tip. This whole manual paste thing has me confused. Some very experienced people use it, but it doesn't make sense to me, especially on the ICs. (Other than BGA's, which I have no experience with.)

I've seen some very impressive pasting on individual IC pads, but by the time they're done pasting just this one IC, I would have been done placing, soldering, and inspecting the chip. I find the pasting useful for the passives, in particular. If I'm hand-pasting, I'm usually hand soldering the IC's, first, and only pasting the passives, diodes, caps, trannys, etc. If those parts all had gull-wing leads, like an IC, I would never hand paste anything. You can touch a soldering iron tip to a point as fast and easy as you'll ever be able to hand apply solder paste.

Even for assembling UQFN leadless protos, I find it most time-efficient to use regular solder. I flux, tin all the pads and "leads," more flux, put the chip on top and then hit it with the rework nozzle. Until I have an X-ray inspection machine, this will probably remain my preferred method for protos.

FWIW, I find 18-20 gauge is good for all around use. And the main problem with the plastic needles is they take a lot of paste to prime, which doesn't all suck back into the syringe when done. So when you're done pasting, you lose a bit when you toss the tip and put the air tight cap back on. (I've tried plugging the needle with pins and/or hot glue, but the paste always finds a way to dry out). But they're dirt cheap in quantity and decrease drag.

[rx8pilot]

Hand solder? Hmm, wonder if you could do that easier with the right iron tip. This whole manual paste thing has me confused. Some very experienced people use it, but it doesn't make sense to me, especially on the ICs. (Other than BGA's, which I have no experience with.)

By 'hand solder', I really mean paste and hot air. For most of my applications, I have to be very careful not to damage a part. Soldering irons VERY rapidly heat up a joint and then it cools very fast because the surrounding material is rather cold. To place or replace a part that needs to be reliable in the future - I use manually applied paste and a hot air system to delicately do the job.

First, if needed, I clean the pads where the part will be placed. Second, I apply the paste. For some parts, I will put a thin line down and for others I will use dots of paste. Next, I place the part itself. The last part is using the hot air system to slowly heat up the PCB, slowly concentrating the heat on to the part until it flows. Then I can slowly ease off the heat. It's not exactly a perfect reflow profile, but it is FAR better than shocking the parts with a soldering iron. For prototypes that only require a short life span, this can be fine. For parts that are going to work long term in the North Pole, it needs to be done gently.

I solder with all available methods. Generally, for delicate fine pitch parts I use paste/hot air.

[KL27x]

it is FAR better than shocking the parts with a soldering iron. For prototypes that only require a short life span, this can be fine. For parts that are going to work long term in the North Pole, it needs to be done gently

If you say so. I would like to see some data on that before I agree.

[rx8pilot]

I am not making this up, but I am not going to dig up any support for it either. There are a number of good reasons that commercial re-flow ovens have very delicately controlled temperature profiles that conform to the recommendations of the solder and component manufacturers. The manufactures of the solder and components have spent huge amounts of money and time learning how to do it. Contact a manufacturer of MLCC's and ask them what happens to iron soldered caps.

 If they simply blasted a ton of heat into the parts until the gray stuff melts - they would end up with all sorts of micro-fractures in the silicon and other areas. They might work on the first day, but the life span will be unpredictable.

[KL27x]

There are a number of good reasons that commercial re-flow ovens have very delicately controlled temperature profiles that conform to the recommendations of the solder and component manufacturers.

Logically speaking:
1.What does this have to do with soldering by iron?
2.Reflow ovens have to account for the most heat-sensitive components, of which there are many which are more heat sensitive than typical IC's. Pitch of the leads isn't a factor.

If they simply blasted a ton of heat into the parts until the gray stuff melts - they would end up with all sorts of micro-fractures in the silicon and other areas. They might work on the first day, but the life span will be unpredictable.

The silicon is on the other end of a very fine gold bonding wire. Gold is a pretty good conductor of heat, but a wire so thin does not transmit heat very fast. I'm not understanding how applying temperature and duration-controlled heat to the tip of the pins (which is the only place it's actually needed) necessarily has less of a margin for error than applying heat for a longer duration, over a larger area of the chip, while waiting for the entire chip and board around it to reach suitable reflow temp. Shouldn't the "reflow curve" of an iron be much more forgiving, acting directly on pad/pin interface as it does? I would think "blasting a ton of heat" is more descriptive of hot air and oven methods. Which is part of the reason reflow curves are so specific, maybe?

Take the gold bonding wire and make it bigger. Let's say that if we increase the dimensions to where the wire is 1" thick, it's now 20 feet long and it weighs 100 lbs. On one end is a heat sensitive square of silicon about a couple yards square and weighs 100 lbs. On the other is a 6' by 1' cross section 1 ton pin. And let's flow it to a huge pad. We can use a finely tuned flamethrower (iron) to heat the end of the pin. Or we can put the whole thing under hot air until the joint flows. Which do you think would have more margin for error?

[zapta]

I got a needle sampler from this seller. Per my request, ge was nice enough to mark the guage of each color so I can reorder by guage #.  Below is the picture I took at the time. Anybody knows if the color coding standard?

http://www.ebay.com/itm/Blunt-end-dispensing-needles-syringe-needle-tips-1-2-30-pcs-14Ga-to-27Ga-/261457346230

[neslekkim]

I bought from ebay, various sellers, and bought from this place: http://www.proto-advantage.com/store/product_info.php?products_id=1200013
All different colors, I was hoping for an standard, but it doesn't seem so..

[Hellmut1956]

A couple of years ago I had the need for a way to solder 3x3 mm Ics with 6 pads. It was a temperature and humidity sensor from a swiss company called Sensirion. As I had already built myself the stuff to pass the circuitry printed on an overhead projector foil I had the need to apply solder paste and solder the devices onto the solder pads without creating bridges between pins while applying the device onto the pads.



I decided to build myself an oven for reflow soldering. I did purchase an oven for pizzas:

https://live.staticflickr.com/4471/37555329470_af54bfcac2_z.jpg

Additionally, I did purchase a multimeter with a temperature measuring function and a sensor that could monitor temperatures above 300° C. As I am sure you all are aware of in the datasheet of a surface mount device there is also a graphic that shows the temperature profile over time:



What is interesting in this graphic is to look at the time a device in the reflow process I allowed to be without harm. First I had the intention to have an electronic circuit to monitor and drive the oven along the limits allowed in time and temperature. I was in a hurry so I decided to test if I could match the profile just by looking at the temperature. I succeeded to learn that it was no problem to just monitor visually the temperature on the display of my multimeter. The critical part of the profile is the time the reflow oven will remain in the peak temperature range. I had no problem staying within the allowed boundaries.

The second aspect that took me even longer investigation and a learning process was the issue of the soldering paste for the reflow. Soldering paste needs to have a recent manufacturing date as it loses important characteristics that make the reflow process fail. So I had to make sure, besides the production date of the charge of paste, I had also to care about the temperature at which it will melt. Suppliers try to cheat you by delivering older paste or paste with less silver content.
The next aspect is related to the needles and the dispenser. The paste is classified depending of the maximum size of the particle in it. Class 5 can be OK. but I did chose to buy class 6, the soldering paste with the smallest size of particles in it. The next aspect is related to the viscosity of the soldering paste when applied. The soldering paste that I got here in Germany comes in a cartridge that is a bit like the dispensing tools you have presented. So if I do remember it correctly the paste had to have a temperature of I think 60°C. At this temperature, it was as easy as possible to apply the paste thanks to its low viscosity.

Finally, as I was managing the reflow process manually and visually I did apply paste to a small PCB, put it into the oven, and watched it to find out when it was melting. The paste not melted was opaque, once the paste melt it was brilliant. The temperature where the paste melted was in the range with peak temperature. To ensure it was properly melting, once I had the visual impact that it had melted, I did count from 1 to 10, switched the oven off, and opened the front door of the oven. So it really was not a challenge to reflow SMD parts. But I do like the dispenser shown in the video, I think I might purchase it later.

[cdev]

Would pine rosin be an appropriate flux to add to old solder paste from the refrigerator?
Do any pastes have both a long shelf life and represent fairly good value?

There are quite a few flavours of paste. The main thing is to use a paste designed for dispensing, not stencilling - the former is more runny and sticky.

Another important thing is temperature - if it's too cold, it won't flow and it won't stick - trying to use it fresh from the fridge is hopeless.

Old paste can sometimes be revived by adding some liquid flux and mixing thoroughly.

I was told by one manufacturer that shelf life is as much to do with moisture absorbtion as temperature, so keeping it sealed, maybe with silica gel, is a good idea. 
I've found EFD SolderPlus paste has a very long shelf-life (many years) , but IME is hard to buy in small qtys.

[Hellmut1956]

@cdev: older soldering paste and soldering paste with a lower silver content are cheaper and can be used if where you plan to use it higher temperatures must be acceptable. if you see my temperature over rime diagram and you see how little time is allowed at a temperature range were soldering paste that is fresh and contains enough silver melts, you see what a critical factor the melting temperature of the soldering paste you want to use.