Odd Stencil Soldering Problem

[Supercap2F]

Hey Guys!

So I've been working on a project that requires a stencil to solder the parts. So after applying solder paste and soldering the board, a lot of the parts had shorted pins and some had hardly any solder on them (circled in red):

From my limited experience (this is the first time I've used a stencil), it looks like there might have been too much paste on the board after removing the stencil. But that would be weird, because I made sure the stencil was very flush with the paste before removing it. I bought the stencil from www.oshstencils.com, it's just their standard 4mil stainless steel stencil. Here's my stencil setup and the board after applying solder paste:


I'm not to sure what went wrong here, but I would really like to know so I won't make the same mistake next time. I have fixed the shorts and stuff with some rework though, and it seams to be working fine now.

Thanks a bunch!
Dan

 

[Alec_t]

Looks as though the solder paste has crept under the edges of the mask apertures. What hold-down method was used to keep the mask closely in contact with the board?

 

[hevans1944]

Did you use a hot-air gun to melt the solder paste, or place the board in a re-flow oven? Can you recommend a re-work station? A re-flow oven?

I've not tried this assembly procedure yet, but it looks like you may have used way too much solder paste on the board, and maybe not have had a firm contact between the stencil and the board as you rolled the solder paste onto the stencil. After apply the paste and rolling it on, did you use a squeegee to remove excess paste from the stencil before lifting the stencil from the board? How old is the solder paste you used? This stuff apparently has a short expiration date.

I have a project that will soon require me to make boards like yours, but considerably smaller, and would really like to learn from your "mistakes" instead of making a few of my own. Thanks for the link to the inexpensive stencil maker.

 

[(*steve*)]

I'll double down on Hop's comments. A good heating profile will fix a lot of your problems I guess.

Having said that, if you managed to get the paste under the stencil or pulled some of it away with it, there is a point where you need to wipe it off and try again.

Examining the board after applying the solder is a must.

I generally apply solder manually to boards I have made (I.e. No solder mask) for prototypes and solder bridges are a thing of the past with my newest reflow oven (which is home made)

 

[Supercap2F]

So I did use a hot-air gun to melt the solder, and I have seen people do it successfully with one. The one I use is off of ebay (like this) and it works pretty good. I'll have to look into a reflow oven though, sounds like it might be quite a bit better.

I thought I had a pretty flat contact between the PCB and the stencil (see pic two of the first post), but that would seam to explain the shorts if I didn't. The solder paste I used was brand new, so I don't think that was an issue. And I did remove all of the access paste from the stencil before lifting it.

Is it better to peal the stencil back, or to lift it strait up? Or does it matter?

 

[(*steve*)]

Do it whatever way doesn't cause any of the solder to stay attached to the mask.

 

[OSH Stencils]

Hey fellow makers, we saw this post and wanted to chime in. This looks like a pretty clear combination of two issues.

1) Paste quality and temperature
2) Application

You have excessive paste release on the pictures provided, this is almost always caused by 2 things.

1) Uneven pressure or multiple passes of paste applied across the stencil. To get the best results, it's always best to have an even distribution of paste across the design on a single pass of the spreader, from one direction. Going back over a design will push paste underneath the stencil creating the globs that you see in your picture which almost always leads to excessive bridges. Think of it like a window squeegee, start at the top, go one direction, one time. The same principle applies to solder paste. It's typically best to start on the side you have the tape affixed to hold the stencil in place, holding the pressure with your thumb or hand on that end of the stencil to keep it from jostling and spreading the paste with the other hand. Keep the spreader at about a 35 degree angle with even pressure and use one smooth motion. (This takes practice, but after 4-5 tries it should fall into place and it'll become trivial).

2) Paste quality and temperature. This is a big one, especially in an environment where you have warmer ambient temperatures. Paste is typically kept refrigerated, and when you remove it to bring it up to ambient temps, you have a limited amount of time to apply it and place your components before it will start to separate. Using a poor quality paste or a paste that is too warm will often create the result you see in your picture.

That said, judging by the image I'm more inclined to believe this was a multiple application/pressure issue as the paste is visibly worse in the lower left portion of the board than in the top right area.

With a proper application you should see almost no paste beyond the copper pad area, assuming you have the stencil aligned properly. If your paste is liquified enough that it easily bleeds across pins upon application, it's time to cool the paste a bit, consider a new paste if it's low quality, or replace it if it's expired. Solder paste typically has a 6 month shelf life with proper care.

Let us know if you have any additional questions.

 

[(*steve*)]

Actually, reading the above post reminds me that I have seen two distinct viscosities of solder paste.

The stuff in large tubs which is made for stencil application seems to be far "thicker" than the stuff designed for pneumatic application (one pad at a time through a nozzle). Could the wrong type of paste be the problem?