Defect of the Month – Non Reflow of Solder Paste

Hello, my name’s Bob Willis and welcome to Defect of the Month. Every month on You Tube we examine one particular type of process defect and provide illustrations, comments and hopefully solutions to point you in the direction to hopefully overcome the issue. If you see this problem in manufacture or some of your colleagues do, remember to subscribe to the You Tube channel, like the video and ding the bell to hear and see more of our Defects of the Month solutions.

This month, I’d like to talk about incomplete solder paste reflow. Now we all know hopefully what solder paste is? It’s a mixture of solder alloy, flux, solvent and a vehicle to allow paste to be printable for a long period of time. Solder paste is being used in millions of kilos worldwide. Solder paste non reflow can be simple when we just haven’t got the temperature right as shown in the examples below

 

So if you’ve got a solder alloy to reflow, you have to be 20-40C degrees over the reflow temperature to allow the products you are soldering to get up to a satisfactory reflow temperature. So that’s not holding the solder paste reflow back, but making sure that the solder paste is at reflow temperature for a period of time to make sure you’re getting good wetting, flux activation of the materials and of course minimizing any voids that might be present on area array packages. So, there are certain things we do. We temperature profile a product to make sure that joints in particular areas are at reflow temperature for the period of time necessary, and we can then judge the quality of the solder joints optically or with x-ray. That’s the fundamental process. If it doesn’t get up to reflow temperature or only just reaches temperature and it’s not there for long enough, then we are not going to have proper reflow and you’ll be able to see this in the joint quality

Sometimes you’ll see particles, spheres of the solder sitting on the surface, not completely reflowed. You might see what I call warts and spots on solder joints, particularly BGAs. This is where the solder place hasn’t coalesced into a solder joint. Similar to the example BGA joint above. Fundamentally if you see a board where the solder paste has not reflowed, it may not just be temperature related. There are two other issues that can also occur. First of all, let’s talk about the temperature related issue. If you reflow a product and you have too long a period of time at the preheat stage or the temperature just below reflow prior to going into reflow. Long soak times you can actually exhaust the fluxing action sometimes called “Graping” and was often seen during the introduction of lead-free, on very small paste deposits for 0201 and 01005 chip components or where nitrogen or vapour phase was not used for reflow. Consequently, all the solder paste particles will not coalesce. They will not join together to create a homogeneous solder joint. I show a few examples above based on my experience with this defect. 

   

The other non-reflow example is where we have reflow temperature and another variable. One of the variables I see is where paste is mistakenly printed onto a circuit board directly after a wipe off or wash operation to remove paste. Solder paste print may not be satisfactory in terms of print quality or volume. Whenever you clean a printed circuit board, particularly if you use a solvent cleaner, you’ve got to make sure that that solvent has evaporated from the surface. The board should be held for a period never printed directly after cleaning. Some solder masks can absorb solvent during cleaning before second stage assembly or a print reject.  

 

So, what potentially happens? You print the board, you place the components, you put it through reflow. Evaporation of the solvent from the surface of the board deactivates or effects the successful reflow of the solder paste. You might see this on specific areas or it may be all over the board surface. 

Most commonly non-reflow, it’s just the temperature is wrong, but just think about that cleaning operation and that impact on the flux material. 

 

If you are running pin in paste, intrusive reflow, sometimes you can get perfect joints and they look good. But right on the tips of the pins, you can see some paste particles have not reflowed and you’re think that’s strange. Well, again, it’s that situation where the solder paste is sitting on the end of the pin as it goes through reflow. It will see elevated temperature for a longer period of time than the bulk of the paste volume in the hole. So in that situation, the preheat and soak temperature just below reflow is having an impact on the flux’s ability to allow the paste on the tip to reflow? 

Always take time to think through logically why a defect might occur and hopefully you can solve your process problems. My name’s Bob Willis with Defect of the Month hopefully you’ve enjoyed this series. Please subscribe to the YouTube channel and you can see many more defect solutions over the coming months