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double sided reflow criteria

#23236

double sided reflow criteria | 3 February, 2003

I know that the accepted (by some, although some say it may be too conservative) rule is anything less than or equal to 30 grams/square inch of surface contact area should stay on the bottom side during the second pass.

How is that surface area measured? Pad size, pad/lead contact surface area, pad/lead contact surface area plus fillet area? Is this nitpicking because of the conservative nature of the number?

This is just to provide engineering with some dfm guidelines in case they need them, not to address a specific situation. Conservative is good, as far as I'm concerned.

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Jon Fox

#23239

double sided reflow criteria | 3 February, 2003

Not to be off-topic, but push engineering for embedded passives and remove parts from the underside all together. Watch the material cost sky rocket, but watch the manufacturing cost sink and well as improve the production cycle by 50%.

Sorry, I wasn't much help, but we use the 30% rule as well. Not sure what they consider in the actual 30% though.

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#23240

double sided reflow criteria | 3 February, 2003

Use the pad mating to lead wetting area.

Check: * Phil Zarrow's site [ http://www.itmconsulting.org ] for a paper. * Bob Willis may have something on his site [ http://www.bobwillis.co.uk ]. He has been very involved in developing this measure. * Fine SMTnet Archives for background.

It seems like adding guidelines like this to the body of knowledge in your DFM project is a very laudable idea.

I'll bet keepouts for PTH components on double sided SMT boards is something you're working-on in parallel with this, eh? ;P

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MA/NY DDave

#23244

double sided reflow criteria | 4 February, 2003

Hi David F

Now I clicked on Phil's site and only got the front page.

How do I or others get to the beef of this particular topic.

Phil is funny and fast

YiE, DDave

This message was posted Add this forum to your site! Click to learn more. the Electronics Forum @

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roertner

#23245

double sided reflow criteria | 4 February, 2003

This rule is very conservative depending on the epoxy you use and the subsequent handling of the board prior to reflow. You should use lead to pad contact area. Also be sure to look at part height and clearance on your flowsolder machine.

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#23248

double sided reflow criteria | 4 February, 2003

Near the top of Phil's front page is thin blue bar with white lettering that extends horizontally across the page. Click on the word "articles" on that blue bar. It will turn a rusty-brown and load another page. On that page, click on the article of interest, maybe start with �Fear and Loathing with Double Sided Reflow Soldering�. [This may not be the article you want. I can't remember. It can't hurt to read it anywho.]

Good luck

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#23255

double sided reflow criteria | 4 February, 2003

"Use the pad mating to lead wetting area."

If I get your drift, we're talking about the minimum lead area (assuming the pad has an area that's at least as large as the lead) required to be wetted by IPC-610.

Hmmmm...that would take some calculations by engineering up front AND a familiarity on their part with 610 re: minimum fillet height requirements for all lead styles that might show up on the bottom. That is, to say the least, an unlikely scenario.

On the other hand, I saw a reference (in the fine SMTnet archives) to an article on SMT (May '98, couldn't find it in their archives) that said 30g/sq." of land area. Now THAT sounds like something definable and easily acquired by designers. Also less conservative.....awwwww, dang.

I thought this was going to be easy...now I find it's actually going to require some work. I think I feel a matrix coming on......

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