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No Lead BGA Hot Gas Rework

#27993

No Lead BGA Hot Gas Rework | 12 April, 2004

Former company we use to replace No Lead BGA's by using dot dispense to apply eutectic solder to pad with same volume the stencil printer. Current company process removes bad BGA, wicks off solder paste then Hot Gas Reflows new No Lead BGA to bare pads. I am wondering about reliability issues. Has anyone had problems with reliability just reflowing No Lead to bare plated pads? Also we use to have no problems using No Lead BGA's installed on eutectic solder printed PCB. Reflow temp was usualy 215 peak with 55 to 65 seconds above 183C. This company want me to adjust Reflow oven temp to 238C because on No Lead BGA is used, when over 250 other componets on PCB are not No Lead and are using eutectic solder. This does not sound correct to me. All cross sections and SEM pictures from previous company showed excellent joints and solder to pad proterties with the No Lead on Euctectic solder using 215C peak profiles. Does anyone have input on this subject?

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RDR

#27997

No Lead BGA Hot Gas Rework | 12 April, 2004

If they are reflowing pbfree balls to bare PCB pads they are getting that component very hot tpo melt the balls and get them to wet to the pads! I would not recommend this process at all for hightemp solder balls, for 63/37 this process is fine. As far as adjusting reflow profile- if 63/37 paste is being used with leaded (pb) components then the profile should be for the 63/37 paste. Hightemp (pbfree) solder balls are for standoff of the component to prevent shorting from excessive collapse, these are usually on CBGAs or other heavy components. Of course with pb free coming around the bend all BGAs will use some sort of pb free ball. I don't believe that the use of these mandates that the ball itself gets reflowed but ensuring that the solder paste reaches appropriate reflow temps for good grain structure, etc... this temp is most likely going to be that 238 deg. that you mentioned. but again, this temp is for the solderpaste and not the component balls. Your previous methods were the correct processes for your product. Personally I would be vary wary of this supplier as it appears that they do not understand what they are doing.

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#28000

No Lead BGA Hot Gas Rework | 12 April, 2004

Russ, These are the pb free components that you mentioned that are coming around the bend. Not the old CBGA's. I use to use CBGA's alot with former company. These are 15 x 15 mm plastic BGA's with 14 mil pg free solder balls going on 8 mil gold pads. We have been doing them for over a year now using pb free solder paste, with major problems. Our customer(one of the largers asic suppliers in Texas)wants us to be able to reflow pb free components to the gold pads on the PCB's. They are already selling Green Board product in Europe. We have no problem doing that, and yes the temps are very high (235C to 238C)and we have fallout from other components that can not take the high temps as other companies have as well. My concern is that this is only one component (BGA) that is on a board with over 250 other components that are not pb free, and all have been installed using eutectic solder paste. We have no problem removing the Bad BGA and cleaning the pad. Even after installing the new pb free BGA on the site the x-ray looks great and SEM pictures show very good joint properties. My issue is joint reliability. I just can not justify running the reflow profile above 217C to 220C just because one BGA is a pb free, but management will not listen to me. I need for other people to agree with my concerns.

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RDR

#28015

No Lead BGA Hot Gas Rework | 12 April, 2004

So if I get this right, your customer wants you to run this product as pb free even though it is not. In order to run this as pb free you need to run pb free paste. If you use 63/37 paste at the higher temps I believe that in addition to the components failing you will also see some bad solder joints from running excessive temps. I believe that if you run 63/37 to 240 C you will form excessive intermetallics at the solder joints. I know others on this forum can explain if this is correct or not. As far as that BGA for rework; I would go ahead and solder it without paste at the temp required to melt the balls, I don't think that that should be a problem since the part should be designed to handle it. (I think you agree with that but making sure) I agree with you that running this board at the high temps without pb free components or solder paste is a bad idea since the other components may or may not be rated to handle those temps. We run 63/37 paste on a lot of pbfree components and have never had an issue. Some TI components for example have been pbfree for sometime now and have been reflowed by numerous parties without a "leadfree" process. Want you may need to do is find the component data sheets for some of the pb parts and show your management that they are not rated for the 235-240 C process.

I hope this helps and some of the smart people on this Forum respond to this also.

Russ

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Ken

#28019

No Lead BGA Hot Gas Rework | 12 April, 2004

I will assume the balls of the device are tin-silver or tin-silver-copper???

There is no reason why you can not run 63-37 solder paste on this device. You will form an intermetallic at the interface structure, however, you will NOT form the 2/3 collapse homogenus solder "ball" (unless you exceed 218 to 221 Celsius). This is the exact same situation as using ceramic bga with Sn10/Pb90 balls. THose will not form balls either, but form the critical interfacial intermetallic required for a good "connection".

I accidentlly did this a week ago....because I did not realize the ball composition....and neither did my customer!!!

Ersa scope and 2D x-ray demonstrated the typical non-eutectic structure that is so COMMON with this attachment method.

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arcandspark

#28040

No Lead BGA Hot Gas Rework | 13 April, 2004

Ken you say reaching 218 to 221 C will give a homogenus solder ball. The pb free solder balls on the BGA reflow at 235 C to 238 C but I have seen when you combin to differnt types of solder say 63-37 and 10-90 you will atually lower the melting point sometimes close to the 63-37's meliting point. Is this what you are saying when you give teh temp 218 to 221C? If this is so I can tweak the profile easily to get up to 223C. Managment here are all these guys that are ledgens in their own minds and even though they have not been working on the manufaturing floor for over twelve years or more, they refuse to believe that you can reflow pb free BGAs to PCB's using eutectic solder. I use to do thousands of cross sections and S.E.M. pictures, so I could see the intermetalics looked great on these kinds of solder joints, thermo cycling and pull strenght testing showed good results as well, the problem in this company is convincing others who always seem to look down at you.

Thanks for you input.

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RDR

#28043

No Lead BGA Hot Gas Rework | 13 April, 2004

Who would these people listen to? Doesn't sound like anybody.

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Ken

#28060

No Lead BGA Hot Gas Rework | 13 April, 2004

I assumed the balls were tin/silver 221C or tin/silver/copper 218C liquiduous....but, this is obviously not the case. It looks like you balls are almost pure tin (mp=238C)

You are correct. This is exactly the same as using 10/90 high temp balls on 63/37 tin/lead solder paste....except you don't have to get to the liquiduous point of the balls (in fact you want to avoid that on ceramics etc.) All you want to do is create an intermetallic at the interface of the liquid solder (paste) and the ball. For this you should reach at least 210C for 63/37...and thats it (effectively you begin to "disolve" the high temp metals into the liquid structure creating the intermetallic connection. This condition is well documented...

This is a simple situation. If this process did not work, they how the hell have we been soldering Texas Instruments devices for over 20 years? Also, AVX produced billions of inductors with silver / paladium finishes in smt as-well...

I see we share the same management team. The believe their own head lines...(snicker).

I ran into this a recently. I was at a lead-free seminar hosted by Intel. They produced a flash memory used in damn-near every cell phone in the world. Many phone manufacturers were transitioning to lead free. Many were not. Intel produced two versions: standard tin/lead and LF. They were interchangeable for functionality but found in the field that some phone manufacturers had substrates and adjacent parts that could NOT withstand the minimum temp requirements for the LF device (when using tin/lead solder paste). So, in effect it was NOT interchangeable but only due to OTHER component restrictions. Hope this helps

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arcandspark

#28065

No Lead BGA Hot Gas Rework | 14 April, 2004

Makes a lot of sense, thanks for the excellent input. The peak temp I am running fo our pb free (238C and approx. 90 seconds above 183 C)is the recommended profile from T.I and also the solder paste manufacture. Not sure exactly what the melting temp is for certain, I will find out.

Ken, thanks again

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Nick Prince

#28122

No Lead BGA Hot Gas Rework | 18 April, 2004

You might like to check ou the SSTC at the National Physical labs, in the UK. They have done a lot of research in mixing PB and PB free alloys. Its high Science stuff but intelligable. The upshot seems to be that if you mix PB and PB Free alloys the resulting joint is MORE reliable than conventional leaded solder.

http://www.npl.co.uk/ei/clubs/

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#28129

No Lead BGA Hot Gas Rework | 19 April, 2004

Thanks, but the site will only allow you to access the information if you attended the conferance. Sound interesting.

arcandspark

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pcb components X-ray inspection