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Profile control parameters

#23080

Profile control parameters | 20 January, 2003

I seem to recall seeing some references to controlling the time above 205�C as a critical parameter, although I can't find it in the archives anywhere other than one reference to a maximum of 45 seconds. What's ideal (typically....I realize different pastes are.....uhhhh....different), why, and why doesn't my paste supplier think it's critical enough to provide even a range? They only seem concerned about the stock slope, soak, and time above liquidus ranges.

DaveF, I'd swear I'd seen you comment on this before, probably more than once, but can't find it in the archives. What can I say?

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Jon Fox

#23082

Profile control parameters | 20 January, 2003

Double check the data sheets for your paste. Just as an example, ou Indium paste has a recommendation for the Heating, Liquidus, and Cooling stagees of reflow. They recommend a linear ramp up of 0.5 to 1.0 degree C. In the Liquidus stage, a peak temperature of 25.0-45.0 (215oC shown on their chart) above the melting point. It also states that it should not remain liquidus more than 45-90 seconds as this can lead to "flux charring, excessive intermetallic formation and damage to the board." For the cooling stage, no more than 4 degree C change per second. If it is cooled down too fast, "the solder joint can be stressed due to high CTE mismatch." look for something like this on your datasheet. Most paste (Kester, Indium, etc..) have this information on their websites.

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#23083

Profile control parameters | 20 January, 2003

Sorry, what I meant was that they don't provide a duration for that liquidus + 20�C value that I've seen bantered about here and on TechNet. They only provide the same stock time above liquidus of 30-90 that they've always used. I'd like to see references to studies that show that the liquidus + 20 is a more pertinent value, or at least an explanation as to why it's a more valuable control parameter.

They do provide recommendations for slope, peak, and soak values too.

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#23084

Profile control parameters | 20 January, 2003

Hi Stevo,

Most paste suppliers say something like, �above 183*C for 45 to 60 seconds.�

The problem we have with statements like that is that they seem to assume that the pads, component leads, and solder paste are all near eutectic 60/40 solder. A couple of years ago that was a reasonable assumption, today it is not.

I prefer saying something like, �In order to get reliable reflow, you need to be at or higher than [Liquidus + 20*C] for about 5 to 10 seconds.� I�m not sure in past [or future] postings that I�ve always said �Liquidus + 20*C�, but it�s been something like that give or take 5*C. Remember, the object of the whole exercise is to make sure ALL solder connections of an assembly receive adequate heat during reflow. Here �adequate heat� varies according to the peculiar solder alloy on each pad.

Consider reviewing IPC-7530 "Guidelines for Temperature Profiling for Mass Soldering Processes (Reflow & Wave)".

If you search on � liquidous plus � in the �Forum� you find other similar comments.

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MA/NY DDave

#23087

Profile control parameters | 21 January, 2003

Hi

You already received good answers.

A thing to remember is the components and the PCB.

How high you go above 63/37's 183C should be limited to create a good solder joint. Too long creates problems since the components and the PCB start to be heated beyond their stable limitations.

As we move into higher solder material compositions like the No Lead varieties this is becoming more of a watch item.

YiE, DDave

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#23088

Profile control parameters | 21 January, 2003

OK, so the issue is with the variability of the alloys on the soldering surfaces and the subsequent variability of reflow temps. Sounds reasonable (actually obvious, now that I think about it), especially on the component side. We only use HASL finished boards, but our components vary quite a bit.

The +20 just seemed to ring a bell, and I have a Technet message on my wall from Werner E. that mentions that, and his comments usually have a ring of truth to them, much like your own. Thanks.....

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MA/NY DDave

#23099

Profile control parameters | 22 January, 2003

Hi

I thought I would come back to this one for a second.

Joe Fox above gave a nice concise note as to why you must raise the temp high enough above the soldering melting temp (liquidus) yet not too high.

You want to go high enough for paste so that the particles can start to coalesce and wetting is good, yet not too high so that intermetallics will form. In the one book I have it lists this as 25-30C above melting. As Joe noted then you want to cool at a good slope to minimize intermetalics.

Reflow Soldering Processes and Troubleshooting by Ning-Cheng Lee (Indium Vice President)

The board, some internal joints, and any plastic parts take longer to get to temperature (Heat Capacitance is lower) thus you want to quickly get them back down in temperature to prevent charring, board delam etc. I also understand that the flux will also char.

For laughs some time let a board stay over a solder pot far too long and watch what happens. Make sure you have a hook on it to catch it. I'm kidding don't do it.

YiE, DDave

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