|Ask the Experts|
January 7, 2021
Tiny Solder Balls After Reflow
We have been seeing tiny solder balls across our PCB's after reflow.
We have baked the boards, cleaned the PCB and stencil with alcohol, used brand new paste, had the oven manufacturer come in and go through our entire oven and have had the solder paste rep investigate the problem.
Nothing yet has resolved the issue. It almost appears the solder is exploding as the balls are found in areas where there is no paste and/or components close by.
Have you seen this before? Any suggestions for how to resolve the problem?
|Expert Panel Responses|
You may have moisture in your solder paste which is causing the solder balls to explode off during reflow. This can happen if the paste is refrigerated and not allowed to get to room temperature before opening causing moisture to condense on the paste.
SME Production Technical Excellence Staff
There are a number of factors that can cause solder balls or solder splatter. Moisture being one which you have eliminated by baking. I recommend verifying your under stencil cleaning process to make sure the stencil is not depositing solder paste randomly around the board. Also, make sure your preheat (ramp-up) is slow enough to allow solvent to vaporize and out gas gradually.
You may also want to see if this is occurring on all PCB types or just this one. If you run multiple products, you can narrow your variables down by running other products with the same paste and equipment to see if they exhibit the same problem. Good luck, I hope this helps.
Manufacturing Applications Specialist
Have you checked you are not using the wrong under stencil wiping roll that is too thick for vacuum then wiping with alcohol which is a thinner, instead of a proper solder paste cleaner. This incorrect thickness roll then spreads the balls across the underside of the stencil, you then insert a PCB and the balls get deposited back on the PCB nowhere near any components that are pasted. Quite a common mistake and worth a quick check.
Alternatively some poor solder paste formulations have been known to explode through the reflow oven. Hope this helps.
Technical Sales Manager
BLT Circuit Services Ltd
The most likely culprit here is the volatile materials in the solder paste. It's not the only possibility, but you have done good work to eliminate some of the others.
If it is the volatiles in the paste, slowing down the pre-heat ramp rate is usually the solution. This will allow the volatile materials to be driven off without rapid outgassing. Your solder paste manufacturer should be able to provide good guidance here. You need to be sure that you are talking with the correct technical contact directly at the manufacturer; the manufacturer's reps are usually not familiar enough with the formulations to provide this level of technical assistance.
Have you tried reflowing a fresh bare board and checking for solder balls? If you see solder balls, then the board is the root-cause. The solder balls could be because of entrapped solder in the vias that are exploding during reflow.
Technical Manager - Europe
One possible cause is the board itself. Air, moisture or even the alcohol you use to clean the board, can absorb into the board between the layers through the edges (delamination), open vias or any through-holes in the board whose plating is incomplete or cracked. These trapped contaminants are is forced out when heated in reflow from between the layers and will erupt from anywhere it can escape. Such eruptions can "jet" gasses in all sorts of direction and push liquid solder around on the board. Also, but less likely, paste flux gasses may do the same thing, as they escape from under large parts that are close to the surface of the board.
If the board is trapping contaminants due to open edges or cracks in vias or through-holes, your only cure is to remake the boards. You cannot bake away trapped air. If the paste gasses are erupting from under components, it may mean you need to reduce the amount of paste or use one that does not have as many volatiles.
Senior Project Engineer
Electronic Controls Design Inc
Not easy to reach conclusions without seeing the product and having a few other details but looking at the information provided I don't see any mention of the thermal profile.
Are flow profile with a fast ramp-up can cause the water and alcohol in the paste to boil off quickly and create the tiny explosions mentioned. "Tiny explosions" was a great description because the gas bubbles literally pop and propel the balls away from the joint.
You will see Ed Briggs and Ron Lasky of Indium use the exact same description in their white paper on defects, causes and solutions:
Suggest printing out the profile and submitting it to your solder paste manufacturer for review. And in the interest of balance, here is a link to a presentation from AIM on troubleshooting:
Heller Industries Inc.
I have more questions than answers and without more information this response is speculative. Here are a few places to start looking:
Technical Marketing Manager
I've experienced the same condition after stencil misprints. First, check your stencil print wash settings and purge the stencil solvent lines to verify if solvent is getting to the stencil.
Second, when not all of the solder paste is removed during the wash cycle, solder spheres tend to lodge in every small crevice they can find in the PCB. Use a solder paste cleaner with a brush for cleaning your PCB's followed by ultrasonic wash. Use the ultrasonic on bare PCB's only to avoid any damage to components.
Senior Manufacturing Engineer
Solder balls can be resulted from a the following processes.
Process Engineering Manager - Electronics
Altech UEC, South Africa
In my experiences I have seen several causes you could check out.
Austin American Technology
This problem requires a true root cause analysis. Changing a lot of things without a plan and a correct data analysis will not fix the problem - I would hate to give another advise that will not get you anywhere - I can think about hundreds of things that can lead to this type of result.
I have experienced this issue before - several times but every time there was another factor that contributed to the result. It is just no way for me to give you are commendation that will help you without more data.
Engineering and Operations Management
For me the first is try to find out the place were are appearing this solder paste that produces solder balls:Domingo Lebron, Magneti Marelli Automotive Lighting
Also note your choice of solder mask may increase the quantity of solder balls in open areas.Bruce Webster, Iridium Communications
Not so much due to brand but more related to the finish. A high gloss finish is more apt to allow solder balls than a matte finish.
The actual mask finish is determined by the customer requirement vs. the process requirements and them find the best paste for appropriate results.
As previously stated in this thread, a DOE is most likely your next move.
It's almost always moisture that is the culprit. Is the paste allowed sufficient time to stabilize after refrigeration? An area to look is simply the paste left on the stencil. What is the ambient RH? How long is paste allowed to sit around before it is changed? Sometimes simple things like shift changes and short line down's to fix feeders can cause the paste to absorb humidity from the room and this problem will come and go like a gremlin.Brad Fern, Entrust Datacard
Make sure there are rules around paste changes that work in your environment. I.E., do you simply keep adding to the paste all day long, or does the stencil get wiped and paste replaced mid-shift? I've seen a lot of trouble caused by $3.00 worth of paste that the operator didn't want to "waste".
Random solder balls can be caused by different factors. If solder balls appear only on the printed area, the root cause can be insufficient flux activity, powder oxidation, flux burn off or fast ramp-up rate during the reflow. If the solder ball appears outside of the printed pad area, it can be caused by spattering (moisture on board or absorption into the paste); poor stencil to pad gasketing or insufficient stencil cleaning.
Director New Product Development
Metallic Resources, Inc
Most often I find the profile needs more temp at the beginning to bake off excess flux. Otherwise at reflow the leftover flux boils and pops causing solder balls.Tom Beall, PCB Assembly
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