|Ask the Experts|
August 18, 2020
Solder Paste Past Shelf Life
We have some solder paste that has been in a sealed box at 4degree Celsius for more than 8 months. The shelf life is 6 months. Canit be used?
What adverse effect might we expect, if any, using solder paste slightly over the shelf life?
|Expert Panel Responses|
Best to follow solder paste manufacturer's recommendations,however, it would be worthwhile to print some and test it before recycling the expired paste. Test for printability, solderability, voids, fillet formation and fillet wetting angles.
If everything is acceptable, then why not use? It is best to repeat using another jar. If any of the attributes fail or if at all suspicious, return it to the supplier for recycling and replacement.
If you do not have internal capability to re-certify the material, contact the manufacturer. They should be able to re-certify it. The concerns are as follows:
Finally, you would need to look at hot slump, to ensure that any viscosity behavior changes have not resulted in a dramatic change in slump behavior.
Solder paste consists of three major elements, solder powder of a give size, flux, and a viscosity agent. All three work together to create therheology of the solder paste. The size of the powder allows the paste to be used for specific size terminations, the flux is selected to reduce the oxides on the materials being soldered and the viscosity agent being able to maintain suspension of the powder within the mass of solder powder within the solder paste mixture. Without the proper viscosity material the solder powder would separate and sink to the bottom of the jar.
The other issue with solder paste is the function of the flux.In plated through hole soldering the flux is used to prepare the component leads and the circuit board prior to soldering. The same applies to flux within the core of the wire solders being used in the industry. However the flux in the solder paste has an extra function, preparing the solder powder in addition to preparing the component lead and pad area.
Since the flux is in constant contact with the solder powder, it is continually acting on the oxides on the surfaces of the solder powder or spheres, and this action reduces the activity levels of the fluxes. This is also why it is always recommended to refrigerate the solder paste to reduce this chemical reaction with the solder powder and give a longer shelf life to the material.
Whether or not the solder paste can be used, would depend upon the type of flux being used, be it either a low activity, medium activity, or high activity flux. The adverse affect would be un-reflowed solder paste,insufficient reflow, solder balls, dewetting, etc.
The recommendations would be to perform a solderability test,slump test and flux activity test on the solder paste itself and see if it still performs to the original standards of the material.
Vice President, Technical Director
Shelf life typically defines the warranty limit for a solder paste. Many remain fully functional beyond the published shelf life. It the solder paste prints or dispenses and reflows appropriately it should be okay.
If either application or reflow looks abnormal you probably should not use it. Typical symptoms of age are poor print or dispense and visual soldering defects such as incomplete coalescence and abnormal solder balls.
Application Engineering Supervisor
If the solder paste appears normal after allowing it to come to room temperature, with no visible separation... it's probably OK to use.There may reduced stencil life, rheology changes and a minor impact on wetting, but if it prints and reflows OK... It's OK.
The real issues are: "I'll save a couple hundred bucks by using expired paste and potentially, increase defects and rework due to clogged apertures, so is it worth it?" Even worse... suppose there's an issue after the build ships to the customer and they start asking questions and you're forced to reveal expired paste was used... that could get really uncomfortable and potentially very expensive.
1st Rule of SMT assembly - Solder paste performance is the foundation of the process - it's the last place one should scrimp.
Technical Marketing Manager
It sounds like the paste has been stored in the correct conditions so I would first test its performance on the stencil and its wetting characteristics. It is a good idea to test against a fresh batch of material.Chris Ward, SolderKing Assembly Materials Ltd
Solder paste is a incredibly crucial part to the process and fresher the paste the better. Look at the time it could take to rework all of your boards or the cost of failure and this may be the decider for you.
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