• State Resolved
  • Locked Locked
  • Replies 4 replies
  • Subscribers 52 subscribers
  • Views 426 views
  • Users 0 members are here
Support feedback
Options
Options
Related

This thread has been locked.

If you have a related question, please click the "Ask a related question" button in the top right corner. The newly created question will be automatically linked to this question.

CC2640R2F: Paste Mask on BGA symbol

John Wright
Expert 1390 points

Part Number: CC2640R2F
Other Parts Discussed in Thread: DRV2605

I'm currently doing a board layout for the CC2640R2F 2.7x2.7 BGA package.

I'm using the PCB symbol YFV0034ALA posted on TI's website.

I've noticed the paste mask pad has a complex shape (approximately square with rounded corners).

This is not so concerning. However, what is concerning is the size of the paste mask pad. The paste mask pad is significantly larger than the solder mask pad, which is in turn larger than the copper pad.

This worries me. It seems like there would be far too much solder, when combined with the solder balls.

Question: Is solder paste required at all with BGA packages?

Question2: Why has the PCB symbol designer chosen to make the paste mask pad area so large?

Here's is another summary of the information, some of it redundant with what I've stated above.

Here are a few notes on my present progress with BGA layout.
Layout is essentially done. Passes all layout software checks (no opens, shorts, track/space sizes and spacings, etc.)
Outstanding issues:

Facts:
1) The solder mask approach is NSMD (Non-Solder Mask defined) - this is where the solder mask is larger than the copper pad, which allows the solder to wrap around the copper pad. The SMD approach is where the solder mask is smaller than the copper pad, which prevents the solder from wrapping over the edge of the copper pad. TI has chosen NSMD...this is their call and I have no disagreements with them. This is the most recommended method across the industry.

2) The paste mask on the TI BGA symbol is larger and shaped differently (complex shape; roughly square w/chamfered corners) than the solder mask pad (which is round), which is larger than the copper pad (which is round). The paste mask on all Non-BGA PCB symbols is not only smaller than the mask pad, but is slightly smaller than the copper pad.

Question/concern: I'm questioning whether the paste mask is so large, it will introduce such a large amount of solder paste (and therefore more solder) that there will be solder bridging. Then there is the question of whether there should be any solder paste at all when soldering the BGAs, which might explain why the paste mask pads are so large, since they will not be used anyway,. But why have any paste mask pad at all...why not just close the pad hole entirely...delete it? Why have any solder paste?

I've brought this up with the layout software folks; there is an active discussion going on right now between myself, the high level support person and the software team. What raised this issue up in the first place is the fact that the software tags (creates/paints?) the BGA paste mask pads a different color than all the other (non-BGA) paste mask pads. This is interesting because the software team doesn't know at this point why this has happened. Could it be there is a size comparison check in the software that triggers the color change as a means of flagging this to be looked at by the layout designer? The software team is scratching their heads at this point.

Regards,

John

  • 0
    Expert 1390 points

    I will mention as an example, the TI DRV2605.

    (see attached)

    Package Outline for YZF0009 9 ball DSBGA

    The drawing for the BGA package shows the paste mask similar in size as the copper pad. The shape of the copper pad is round, the shape of the paste mask opening is square with rounded corners. They show both NSMD and SMD dimensions for the solder mask, so there's no real clue as to whether the drawing of the paste mask opening applies to the NSMD or SMD case. The corners of the paste mask opening extend beyond the copper pad. But the size of the paste mask opening is very close to that of the copper pad.

    Can someone from TI say whether the symbol YFV0034ALAL has proven success in any PC board builds with the cc2640R2F 2.7mm x 2.7mm BGA?

    Regards,

    John

  • +1 in reply to John Wright
    TI__Guru**** 171839 points

    John,

    Please apologize for the huge delay; your thread escaped our radar.

    We talked to a packaging expert and he provided additional information regarding the DSBGA package as below:

    This device is a DSBGA with package designator YFV0034A.
    The solder ball are 0.25mm in diameter and 0.21mm height.
    The recommended PCB land pad is 0.23mm round.
    Recommend using NSMD with 0.05mm gap (0.33 mm SMO).

    The recommended stencil aperture is 0.25mm SQ with round corners and stencil thickness is 0.1mm
    The 0.1mm thick stencil is the common thickness for other     component on the same PCB due to customer product application.

    The 0.25mm square stencil aperture is required in order to meet the area aspect ratio to get a good solder paste printing quality.

    Reducing the stencil aperture will result in insufficient solder Printed onto the PCB.

     We have not experience any solder bridging issue during our internal qualification.

    Please let us know if this clarifies your questions regarding the solder paste pattern of the YFV0034 symbol.

    Best regards,

    Rafael

  • 0 in reply to desouza
    Expert 1390 points

    Rafael,

    Did you really mean a truly square stencil aperture?

    I have looked at the TI drawing of the 2605 BGA (see attached), and it uses a square with rounded corners.

    Is it better to used a true square or should I put the same rounded corners as the 2605 BGA?

    Package Outline for YZF0009 9 ball DSBGA

    Sorry, I did not mean to click on the resolved button, yet!

    Regards,

    John 

  • +1 in reply to John Wright
    Expert 1390 points

    Thanks Rafael,

    This is very thorough...I realized you did specify the rounded corners in the statement above.

    Many Regards,

    John