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I am using Tin-Lead solderpaste to make my prototype circuits. This is the exact solder paste:

ALPHA OM-5100 (https://alphaassembly.com/Products/Solder-Paste/OM-5100)

    Alloy: 62Sn/36Pb/2Ag, 63Sn/37Pb and 62.8Sn/36.8Pb/0.4Ag (NT4S, Anti
    Tombstoning Alloy)

    Powder size: Type 3, (25-45 µm per IPC J-STD-005) or Type 4 (20-38µm).

 
I use a stencil to apply the solderpaste. Next, I place all the components one-by-one.

The solder paste contains flux. However, I'm worried the flux evaporates while I'm placing the components. It generally takes several hours - up to 8 hours for my latest PCB - to get all components in place.

I've got the impression that the solderpaste doesn't "flow" sufficiently. For example, I would expect slightly misaligned resistors and capacitors to straighten up when the solder melts. But that doesn't happen.

Another example: Imagine some solder paste that isn't applied very good:

enter image description here

During the reflow process, I would expect it to flow like this - as if it is attracted to the copper pads and repulsed by the naked FR4-material or coatings(where no copper is present):

enter image description here

But that doesn't seem to happen either.

 

Should I spray some flux onto the board before I shift it into the reflow oven? If yes, what spray would you recommend?

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  • \$\begingroup\$ Covering the whole board in flux will probably only serve to make a big mess. \$\endgroup\$ – Tom Carpenter Jan 24 '18 at 18:03
  • \$\begingroup\$ And blow the 0402's off. One point : the flux doesn't evaporate. The solvent containing it does, leaving the actual flux where it is. \$\endgroup\$ – Brian Drummond Jan 24 '18 at 18:13
  • \$\begingroup\$ I think you should rule out other issues. Do you need to clean the bare fabs better? How were they finished and how long have they been stored? Did you profile the oven temperature? Maybe it is not hot enough and the solder doesn't melt thoroughly. \$\endgroup\$ – mkeith Jan 25 '18 at 7:07
  • \$\begingroup\$ The pads have gold-finish. The PCB's were stored a couple of weeks in a plastic sealing. The oven temperature profile is good, I believe. 120 secs above 188deg and also 70 secs above 220deg. \$\endgroup\$ – K.Mulier Jan 25 '18 at 7:32
  • \$\begingroup\$ OK. Two weeks for gold finish should be fine. \$\endgroup\$ – mkeith Jan 25 '18 at 18:35
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The flux does evaporate and it can affect the solderabilty of the board. Typically there might not be enough wetting and the solder doesn't flow as well.

However, I do my own prototyping and the larger boards sometimes take an entire day just to stuff all the parts. This leaves the flux on the board to evaporate for hours. We however do not have problems as you described above, I'm willing to bet that your temperature profile is not as hot as it needs to be. I would throw a thermocouple and see if you can measure it.

It probably wouldn't hurt to throw on additional flux on the board before you put it in the oven, I wouldn't spray it as this would leave a no-clean residue over the board. I would match what you get from your paste supplier and get the flux from them.

If you still have problems you might want to switch solders, I've never had a problem with kester solder flux. I have also been impressed by Qualitek solders (not associated with them, just really like them and the price) but I have only used lead free.

Another thing you might want to consider is switching to a water soluble solder, which you simply clean of your board with a warm spray of deionizied water (assuming you don't have many components with a high moisture sensitivity level, or you can put those on afterwards)

As a recap, I'm willing to bet you need more heat, but you could just have bad solder flux in your paste. You can add flux, I've done that before, but the trade-off is residue and cleaning. You have many options to fix this problem, it really depends on what your components can tolerate.

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  • \$\begingroup\$ Thank you very much! I've got two questions. (1) You say "It probably wouldn't hurt to throw on additional flux on the board before you put it in the oven, I wouldn't spray it as this would leave a no-clean residue over the board.". But if I don't spray it, what other means do I have to apply it? (2) How do I know if a particular component can get damaged by water (I suppose that's what you mean when you refer to "high moisture sensitive components")? \$\endgroup\$ – K.Mulier Feb 6 '18 at 18:11
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    \$\begingroup\$ @K.Mulier: 1) Dab with a brush or pen. 2) Read the datasheet. \$\endgroup\$ – Ignacio Vazquez-Abrams Feb 6 '18 at 18:36
  • \$\begingroup\$ Well, usually you put flux in a bottle like this. Check out technitool or other suppliers. It would be time consuming to apply it where its needed, but hey you already placed those components so a little more time with your neck cranked toward the bench won't hurt. I would rather apply it to just the solder flux than to the whole board \$\endgroup\$ – Voltage Spike Feb 6 '18 at 18:43

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