|Ask the Experts|
October 12, 2020
Solder Paste Life on the Stencil
Our solder paste has a specified life for exposure on the stencil at 10 hours. We run our stencil operation continuously for 40 hours, adding 250g of new paste every 2 hours.
The addition paste thus becomes mixed with the existing paste. After the first 10 hours of manufacturing, do we need to remove all the paste on the stencil and scrap it?
Currently the practice is we keep adding new paste until the end of the 40 hours of continuous production.
|Expert Panel Responses|
It is completely dependent on paste type, we have several types where you change half of it after four hours and it lasts for 7 days, so suggest you trial first and see for yourselves as it may just be fine.
Technical Sales Manager
BLT Circuit Services Ltd
Yes, you need to remove the solder paste on a regular schedule, possibly every 4 hours, and clean the stencil either manually or with a machine.
If you don't clean your stencil your apertures will close and the proper quantity of solder paste will not be put down. Your solder paste supplier will provide you with this information.
Also, you need to watch your room temperature and humidity.
You probably don't need to change it. I'll make the assumption that your 250g additions represent about half the volume on the stencil. You will have made a total of five of these additions at the 10-hour point. After the first addition, you'll have half of the original paste left. After the second addition, you'll have one quarter of the original paste left, and so forth. So after five additions, you'll have only 1/32, or about 3% of the original paste left.
If your addition is actually only a third of the total paste volume, you'll have 13% left, and if it's two thirds of the volume, you'll have less than half a percent left, so as you can see the replacement ratio makes a big difference. The more paste volume that is on the stencil, the harder it will be to replace it.
The environment also plays a role. If the temperature rise inside the printer is significant, the paste may age prematurely. If the environment is well-controlled, you may actually have longer than 10 hours.As they say, the proof of the pudding is in the eating.
Only your print quality can tell you whether the paste has aged too much. You normally be better off with periodic additions than with wholesale replacement,since you will have less dramatic changes in paste behavior when replacing only part of the paste.
My recommendations are:
Simplicity is always the way to go in my opinion.
We opt for even a shorter time than recommended.
The issue with solder paste is that the flux, which acts as a binder in addition to the soldering benefits,evaporates out of the paste over time due to the granular structure. This leads to solderability problems and even clogging of stencil apertures resulting in mis-prints.
Simply adding additional paste will not refresh the existing paste where the flux has evaporated, and in fact can result in diluting the active flux across dry paste reducing its effectiveness overall.
We run timers for paste on stencils and when it goes off, the stencil is cleaned off and fresh paste is applied.
Director of Marketing
Since the original solder paste has been mostly replaced with the fresh paste, you do not have to change the paste after 10 hrs. unless the printability or the reflow properties of the paste start to show abnormal behaviors.
Director New Product Development
Metallic Resources, Inc
My suggestion is to perform a test:Domingo Jose Lebron Berdugo, Magneti Marelli - Automotive Lighting
- Verify how many time you have to add new solder paste
- Every time you have to add new solder paste, first remove the old one, mix and put it again
- Check every hour the evolution of SPI data and AOI data
This will provide you information about your process, and will help you to know when you have to scrap the solder paste over the stencil.
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