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Gold plated board, with csp's and 0603 with no clean apetures

Abelardo Rodriguez

#21760

Gold plated board, with csp's and 0603 with no clean apetures | 26 September, 2002

Hello everyone out there in the SMT world. I have a dilema I'm currently running a board that has 3 csp's with a .75 mm. ball pitch and .3mm ball size. And 2 qfp's-160. The stencil is 4 mils thick and I'm using a 63/37 solder paste. My reflow profile temerature average is 215 deg. celsius and a ramp rate of 1.50, the issue with the board is that the solder connections look really grainy and my Q.A. is rejecting it for cold solder. ( Don't ask why shininess on a connections determins if the connection is good or not ) The thing that I need help is this grainy appearance on the connections is it gold embrittlement or cold solder. I was told that I might be drying the flux out of the solder before it can reflow. I've read that increasing the temperature in the liquidous zone will solve this. But when I do this I get solder spash all over the board. What I did to increase the temprature is a difference of 5 degrees and to stop the flux from evaporating I increased the speed by 2 in. My thickness on the solder paste deposits is around 5 mils. I orinally created a reflow profile and ran a thermal analyzer to determin the appropriate temperatures. It looked very good. Now I'm getting this problem with the appearance and I don't know what it is. Is there somewhere a picture or some kind of aid to help me visually distingush the difference between the 2 issues (gold embrittlement or cold solder ) and how do I get rid of this problem of the grainy appearance.

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Jim M.

#21765

Gold plated board, with csp's and 0603 with no clean apetures | 26 September, 2002

unsure of what solder spec. your working to?.IPC/EIA J-STD-001C, Section 9.2.4 clearly states dull, matte, gray or grainy appearing solders are accpetable depending on your process and if the acceptability of the neaxt paragraph is met.IPC 610 and J-std-001 go hand in hand.

Maybe something in this web link http://www.bobwillis.co.uk/defect_browser/index.htm

Jim M

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#21772

Gold plated board, with csp's and 0603 with no clean apetures | 26 September, 2002

Both you and a previous poster are correct. Grainy / shiney solder are not a good indicator of a proper connection.

Your quality guy is asking the correct question, though. => "OK. If the connection doesn't look like I expect it to look, how do I know that the metallurgy is correct?"

Comments are: * There are no pictures that will get you though this. * You should expect that gold that is dissolved in Pb/Sn solder to make that look solder grainy / dull, whatever. * Gold embrittlement can occur in connections like this. [You didn't tell us, but we assume the solderability protection on your board is ENIG.] You need to be concerned when the gold is over 3%. [Search the fine SMTnet Archives for background.] * We've heard goofy stuff like drying out the flux before. Listen, if you are getting a smooth solder flow, good fillets, intermetallics at the nickel tin boundry, and all that; your recipe is fine. [You created those solder balls during preheat. It was either too hot or too cool, before going to soak.] * Gold increases the liquidous temperature of Pb/Sn. So you need to sit above liquidous plus 20�C for maybe 5 seconds longer than you would with a HASL board to be sure the gold has dissolved in the solder.

OK, back to the quality guy. The only way you can tell what's really going on in this solder connection is metallurgical analysis. There are good labs listed in the fine SMTnet Archives.

On final note: Get used to this discussion with your quality guy. As bare board and component solderability protection change, in response to the Eurolanders no-lead foolishness, there will be more and more 'exotic' alloys on your board.

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Eric

#21816

Gold plated board, with csp's and 0603 with no clean apetures | 2 October, 2002

Is there a steadfast rule of thumb for a good solder joint on an Enig Gold board. I come across this number 217C when discussing BGA profiles. Is this a published number? I sure could use a reference. Anybody feedback is appreciated.

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Steven

#21817

Gold plated board, with csp's and 0603 with no clean apetures | 2 October, 2002

Have you had your solder pot tested? I find that levels of contaminants can rend a solder joint that dull look.

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#21818

Gold plated board, with csp's and 0603 with no clean apetures | 2 October, 2002

The steadfast rule of thumb is: In order to get reliable reflow, you need to be at or higher than [liquidus + 20�C] for about 5 to 10 seconds, rather than those old "60 seconds above 183�C" guideline. This provides for setting your peak based on the actual alloy you have selected that is the mixture of the component finish, solder, and solderability protection.

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

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