Printed Circuit Board Assembly & PCB Design SMT Electronics Assembly Manufacturing Forum

Printed Circuit Board Assembly & PCB Design Forum

SMT electronics assembly manufacturing forum.


Bob Yost

#14921

Solder paste with BGAs? | 22 July, 1998

My firm has just begun it's first PCB that will use BGA packaging. I have been reading up on the processes involved with BGAs and was wondering if you need to apply solder paste to the pads prior to applying heat and mounting the part to the board. Some application notes I have read instruct to use solder paste, while others do not. I suspect that it depends on the chemical makeup of the solder balls of the BGA. Thanks for your help Bob Yost ryost@sed.stel.com

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Justin Medernach

#14925

Treading on dangerous ground | 22 July, 1998

| My firm has just begun it's first PCB that will use BGA packaging. I have been reading up on the processes involved with BGAs and was wondering if you need to apply solder paste to the pads prior to applying heat and mounting the part to the board. Some application notes I have read instruct to use solder paste, while others do not. I suspect that it depends on the chemical makeup of the solder balls of the BGA. | Thanks for your help | Bob Yost | ryost@sed.stel.com Bob, You definitely want to apply solder paste, no matter what the makeup. You need the flux more than anything. If you are doing a Ceramic BGA with Hi-temp sphere's, use an .008" foil thickness. This will give adequate volume for a reliable solder joint. If you're doing PBGAs or anything with Eutectic Spheres, use a .006" foil. The only time you probably wouldn't use paste is if you are REWORKING a BGA with Eutectic spheres. Even here, you have to apply paste flux to the solderable surfaces. Regards, justin medernach

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Wayne

#14924

Re: Solder paste with BGAs? | 23 July, 1998

| My firm has just begun it's first PCB that will use BGA packaging. I have been reading up on the processes involved with BGAs and was wondering if you need to apply solder paste to the pads prior to applying heat and mounting the part to the board. Some application notes I have read instruct to use solder paste, while others do not. I suspect that it depends on the chemical makeup of the solder balls of the BGA. | Thanks for your help | Bob Yost | ryost@sed.stel.com

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Russ Miculich

#14923

Re: Solder paste with BGAs? | 23 July, 1998

Are you using a HASL board or an organic protection on the pads? This will help determine as to whether paste is necessary or just a very active/tacky flux should be used. Tight process control will also help in the reflow oven. Many users don't want more solder paste than is already there in the form of the solder spheres and they use the flux only to help control the possibilty of bridging. As you know, inspection, test, and rework of the BGA is difficult if solder balls are entrapped or if bridging should occur. Remember that just enough solder needs to be present between the ball to pad interface as when the solder sphere flows, the weight of the component will allow it to settle into place properly. Excess solder does not have a "wick-up" path on a BGA be it ceramic or Plastic. Hope this helps. | My firm has just begun it's first PCB that will use BGA packaging. I have been reading up on the processes involved with BGAs and was wondering if you need to apply solder paste to the pads prior to applying heat and mounting the part to the board. Some application notes I have read instruct to use solder paste, while others do not. I suspect that it depends on the chemical makeup of the solder balls of the BGA. | Thanks for your help | Bob Yost | ryost@sed.stel.com

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Gestapo Allen

#14926

Re: Treading on dangerous ground | 23 July, 1998

I think that in general it's best to deposit paste in order to increase the part standoff. This helps improve the fatigue-life of the joints by providing more strain relief. But I know from experience that it's tough to deposit paste during the rework process. We tried using mini-stencils and found that technique difficult. Currently, we're just using a no-clean flux paste for rework (we apply it to the solder spheres rather than the board). If anyone has developed a good method of depositing paste during rework, I'd like to hear about it. I'd also like to know of any technical papers that address the fatigue-life of joints with and without solder paste. Is the effect big or small?

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Bob Willis

#14922

Re: Solder paste with BGAs? | 24 July, 1998

Yes you need solder paste for BGA. Have a look at the Free BGA documents on my home page which you can download. They cover design, rework, and general assembly www.bobwillis.co.uk | My firm has just begun it's first PCB that will use BGA packaging. I have been reading up on the processes involved with BGAs and was wondering if you need to apply solder paste to the pads prior to applying heat and mounting the part to the board. Some application notes I have read instruct to use solder paste, while others do not. I suspect that it depends on the chemical makeup of the solder balls of the BGA. | Thanks for your help | Bob Yost | ryost@sed.stel.com

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Terry Burnette

#14927

Re: Treading on dangerous ground | 28 July, 1998

| I think that in general it's best to deposit paste in order to increase the part standoff. This helps improve the fatigue-life of the joints by providing more strain relief. But I know from experience that it's tough to deposit paste during the rework process. We tried using mini-stencils and found that technique difficult. Currently, we're just using a no-clean flux paste for rework (we apply it to the solder spheres rather than the board). | If anyone has developed a good method of depositing paste during rework, I'd like to hear about it. I'd also like to know of any technical papers that address the fatigue-life of joints with and without solder paste. Is the effect big or small? Using solder paste for PBGA's, increases the standoff height of the package by about .002". On 1.27 mm pitch PBGA's which we temperature cycled from 0� +100�, the packages which were soldered with solder paste, had about a 15% longer life than those packages which were attached with just flux.

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