Today's question is from Ray W, in Andover, Massachusetts. "What is the maximum allowable time for a PCB after it leaves the screen printer but before reflow? We're using water soluble lead-free paste; PCB is densely populated with over 10,000 apertures, and it's 90 mils thick."
What your asking about is something called "Tack Time" and here are our tackless comments on Tack Time. It's really dependent on two things, one is the chemistry of the solder paste and basically this is inherent with the particular manufacturer. The other factor is the relative humidity and temperature in your facility. Temperature is particularly critical with water soluble because typically the solvent mechanism is isopropyl alcohol based so they tend to evaporate a little bit quicker. The actual tack time you should be able to find in the specifications from the solder paste company.
But don't trust them.
That's right. There is an easy test to determine this. This is one of the tests we do at ITM, when we do our solder paste evaluation testing. Our test procedure for tack test goes as follows: What we do is we'll take a board, and it doesn't even have to be a live board because we're very economical when it comes to this, hey, "lean testing". What you do is you take a real pattern, and go ahead and print it on a board and with the solder paste in question.
What you're going to do next is to populate it with approximately 100 components relatively low mass components - passives and discreets. Hey, if you're doing MELF, put down a MELF, too. Maybe some small simple ICs like and SOIC-8, SOT transistors, etc...
Use your pick and place machine to populate it with and with the proper parameters, including getting the right depth of the component in the solder. This is real life, so you should do it in the environment you're doing this in your production environment with the typical relative humidity and ambient temperature.
Now, you've taken the board, printed it and populated it, as we described, and now at T-0, time zero - immediately after populating it, you take the board, turn it 90 degrees so that it is perpendicular to your work surface, and, holding it about an inch above the work surface, release it so it kind of taps the board, Now hold the board upside down, parallel to the work surface. Nest you count how many components fall off!
Now your immediate reaction is: "Well that's easy, they're all going to fall off or most of them." But the reality of it is that there's pretty good tack. Much to your surprise very few if any components will fall off. Now that's at T-0, and you're going to repeat the test at one-hour intervals. So after one-hour has passed, you do the same thing, hold the board 90 degrees and an inch above the work surface, and let it drop to create a tap. Hold it upside down; count how many components fall off. Go out as many hours as you feel necessary. Usually we don't do it anymore beyond four hours; although, we have in the early days of solder paste testing we've gone out as much as 24 hours. I can tell you that we've tested pastes, both no-clean and OAs that even after 24 hours have not had a single component fall off.
Yeah, the solder paste manufacturers are aware of this and they certainly try to give you a good tack time when they formulate it, but it's depending on the other properties, some pastes have better tack times than others.
So try this test at home, Ray, and everybody else out there. It's a good way to benchmark the paste you're using now, and it should be an essential test when you do any solder paste evaluations.