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I recently purchased Kester EP256 solder paste. This is the first time I am working with it, and placing it on the PCB is very difficult. I do not know how viscous it should be, but it seems to be too dry - it virtually does not stick to the board, or the components. It almost looks like a fine wet send - it sticks to itself, but not to anything else. I was expecting it to be more like a toothpaste.

I know, it is difficult to judge the quality of the paste by my description, but how is the paste supposed to be? Also, is there a way to dissolve it, or rather, make it more sticky?

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  • \$\begingroup\$ I believe I would try contacting Kester Customer Service \$\endgroup\$ – Tut May 28 '14 at 20:13
  • \$\begingroup\$ @Tut I did. No reply yet. Waiting... \$\endgroup\$ – Nazar May 28 '14 at 20:18
  • \$\begingroup\$ Also, there is some info in the Kester Knowledge Base under "Flux" -> "No Clean Flux" -> "Flux Thinning" & "Flux Test Kit", but I have yet to find their recommendation for thinner. \$\endgroup\$ – Tut May 28 '14 at 20:30
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I've found that thinning it with a little IPA (isopropyl alcohol) works very well. I've got plenty, as it's main use is for cleaning the flux from PCBs.

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  • \$\begingroup\$ I tried that. It does dissolve it well, and if you add just the right amount of IPA, it has a 'good' viscosity. So, I waited while the IPA partially evaporated and tried to solder it. The remaining IPA in the solder expands simultaneously and tears apart the solder spreading the dust across the board, like miniature bombs. Did not work well, so far. \$\endgroup\$ – Nazar May 28 '14 at 19:36
  • \$\begingroup\$ Are you reflowing it or trying to hand-solder with it? Given your description of the behavior, I think you want "thin-apply-dry-solder" as a process sequence - once applied, it does not need to be "liquid" or even "pasty", so a slow drying/pre-heating step before raising the temperature to reflow would likely help. \$\endgroup\$ – Ecnerwal May 29 '14 at 2:00
  • \$\begingroup\$ IPA worked OK for me. It looks as though it depends on the paste supplier. \$\endgroup\$ – Leon Heller May 29 '14 at 9:02
  • \$\begingroup\$ Yep, I kept the temperature at about 150C to evaporate the IPA and after about five minutes increased it to melt the paste. It worked well. In fact I was able to dilute the paste to the viscosity that would allow me to use very fine needle for 0.5 pitch pins. \$\endgroup\$ – Nazar Jun 4 '14 at 13:11
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    \$\begingroup\$ You actually don't need to put in the effort to individually put paste on individual pads for fine pitch components. I use the smear technique - just smear the paste on the row of pads and throw the QFN on top. I use a blower though so I can tap the part with a tweezer and flatten the chip if a solder blob forms underneath it. Once it is in place I give a slight sidewise tap. If the chip snaps into position then all is good. I then let the solder harden and run a soldering iron along the edges of the chip to remove any excess solder and to make solder contact with exposed side pads if any. \$\endgroup\$ – Dojo Jul 25 '16 at 20:50
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Some pastes are thixotropic, which means that they soften up when subject to shear forces. In an assembly house, when the paste is applied by squeegee (through a stencil), this is enough to temporarily soften it and have it stick to the board.

Another option is to simply stir a small amount, then apply it manually after it softens.

Also, solder paste has a finite shelf life and should be refrigerated. There should be a "Use By" date and temperature guidelines on the packaging.

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You can increase the adhesion between solder paste and your circuit board by adding flux. I would recommend a gel flux. Things will get messy quick but I am confident you will be able to get the consistency you want.

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I have had some experience with this and your solder paste is probably old. It appears to dry out, but that is not exactly what is happening. The solder is made up of tiny little spheres of solder with, an incredible amount of surface area. This mean that there is a lot of reaction with the solder and the flux that it is mixed with (refrigeration can slow this down some maybe 50% more life, 4 months?). Even if you get the solder back to the correct consistency using a nonvolatile liquid such as liquid solder flux, it will still have too much oxide in it that will cause the solder to not work correctly. The solder flux carries the oxide under the components and can cause some resistances around your component connections. This will cause all kinds of problems especially if your circuits are high frequency. This is an aggravating problem and the best solution is a good solder paste that is from a volume dealer. I recommend just buying what you need. If you are going to use a guide template then use solder in a small jar and use an old credit card to spread it (then using a reflow oven is great, I use a Black n Decker toaster oven with good success). If you are hand soldering an SMD component here or there, then you should use the syringe a 1ml syringe is best but use a 5 or 10ml syringe to load it. If you have a steady hand then you are in and if you have a microscope even better. First put the solder paste on the pads and then use an needle or dental pick to carefully move excess on to the pad or remove it. Place the part using tweezers or vacuum pick tool. Then with a needle hold the part in place, touch the tip of the soldering iron to the edge of the pad on the board that you are soldering. The solder will flow quickly and right on to the component then do the other side of the component or opposite corner of the component if multi-pin. I would not use dry solder on SMD components because it is too slow. Remember if you are using solder paste, it will already be under the component connection and your solder is already in place so you have a free hand. Imagine hold the part, the solder and the iron... All of the information here is from experience. I even tried thinning with IPA.

On some safety notes soldering with SMD solder paste especially old solder paste creates a lot of very toxic fumes, much more than regular soldering. Good ventilation and a fan are necessary. Solder paste with IPA in it can cause lead to pop into your eye. And last, this stuff would be deadly if it were ingested by a child (or an adult for that matter) Hands dirty with this stuff will get on your sandwich too! so wash up often.

The answer is get new solder paste. I thought that my whys were useful.

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    \$\begingroup\$ This is a good answer, but it was pretty difficult to read the wall-of-text. I suggest that you remove everything about how to use solder paste, and just focus on the issue at hand - the viscosity. If you want to keep it, maybe break the text up in sections? \$\endgroup\$ – pipe Jun 8 '16 at 4:21

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