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DS90LV028A: Accounting for Soldermask and Paste Thickness

Aaron Fleming
Prodigy 10 points

Part Number: DS90LV028A

Tool/software:

Hello,

We are working with the WSON variant and this is our first time working with a Pullback LLP (Leadless Leadframe Package).

We are having trouble with the part wandering off placement while traveling through our oven. The height of pads off of the bottom of the part body can be quite small (0.0 to 0.05mm) and we are wondering if the soldermask under the part between the pins can prevent the pads from making contact with the board's pads and paste (4mil thickness).

If this is an issue, would pre-tinning the PCB's pads to add some initial extra height help us avoid the soldermask or would that introduce other issues? What about removing the soldermask entirely from under the part to allow the part to lay flat on the pads?

Best,

Aaron

  • +1
    TI__Guru 54042 points

    Hi Aaron,

    We generally do advise to keep the solder mask to avoid issues of solder bridges forming between two or more pads - we usually do suggest non solder mask defined:

    Also just for clarity I am adding our suggested land pattern:

    So I'd advise the solder mask be kept - removing it can be done but the risk of solder bridges forming does increase. The preferred solder mask application does offer benefits of more accurate pad placement, better solder joints, and better adhesion - this type of solder mask definition is usually what is used in high density electronic applications - where you may have very small pads including things like BGAs. 

    I don't know if the tinning would be helpful - it may possibly help - I don't see a huge concern in trying it to help it adhere to the board better - it probably is a better idea than going without solder mask though. 

    So in conclusion:

    1. Ensure the solder mask is non solder mask defined (NSMD) as shown in the attached pictures. Also just in general it seems that green solder mask is generally going to be preferred on these type of pads - manufacturers seem to be able to do a bit more with green solder mask compared to other colors. 

    2. If you have already done step 1 - I would then try tinning the pads to hopefully achieve better adhesion as that shouldn't introduce the potential problems that no solder mask is going to bring. 

    3. If all else fails - removing the solder mask is possible - but with the small pitch of the pads on this part + a thermal pad the chances of solder bridges cannot be neglected. 

    Please let me know if you have any other questions and I will see what I can do!

    Best,

    Parker Dodson

  • 0 in reply to Parker Dodson
    Prodigy 10 points

    Thank you for the help. I do have a different related question.

    Per AN-1187, Figure 18 and Figure 19, it isn't recommended for us to extend the board's pads and paste past the edge of the body for pullback packages, but that then prevents us from exposing the pad for rework. If we extend the pad (say double the length outward to 1mm), but keep the paste in the same location (the inner half of the new pad), will we have the same issue as portrayed in Figure 19? Would the solder paste wick out to the outside of the board's pad before it has a chance to attach to the pad of the part?

  • 0 in reply to Aaron Fleming
    TI__Guru 54042 points

    Hi Aaron,

    Yes - if you extend the pads outward from the package you still run the risk of the weak solder joint because the paste will generally move across the pad and could cause the weak solder joints. This is a trade off of using this type of package; they are great for high density applications but rework is challenging in case of mistakes. 

    Please let me know if you have any other questions and I will see what I can do!

    Best,

    Parker Dodson