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  • SOLDER PASTE STENCIL

    Hi Guys,
    Ordering a solder paste stencil, I got some issues and questions, would appreciate you time if you take a look and give answers:
    - We ordered .125mm or 125um thickness stencil as recommended, but looks that with 125um thickness amount of paste on pad is not enough, since board will need to pass some vibration test. So there is some fear that components will not stand vibration. What is solution, should we order thicker stencil? or change pate?
    - In ordered stencil, when you look at pad shape especially big components like D case tantalum caps, it is clear that pad shape is changed by adding a small triangular to inner side, what is reason to do this sort of modifications? attached is a sample photo,

    Click image for larger version

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    I would like to pad shape change and we asked them to do so, but result is different? Is there any IPC standards (or part of it) defining stencils?
    - In designs with different variant, what happens to stencil, different stencil per variant? I need DNP components with no pate on pad, any suggestion?
    - In general, what design rules we should obey to make a design pick-and-place machine friendly?
    - Regarding angel by which trace enters pad, is there any recommendation or limit which may affect assembly or test?

  • #2
    Hi Johnson,

    Normally our assemblers deal with ordering and adjusting the stencils for us - we only provide them with the PCB data. Because of that I don't have much experience with stencils. I know that its thickness determines how much paste deposit you will get on the pads. I don't know what happened to the shape.

    What is the difference between your variants? If it's just DNP components, I don't think it is a problem to have solder paste on them. They will need to be hand soldered if you ever need to populate them, so having some solder on the pads already is not a big issue.

    For the pick and place, it is good to use properly spaced courtyard drawing for all components in your libraries, which helps to keep the components at the appropriate distance between each other. I know that you shouldn't have BGA packages close to each other - 1 mm package to package is a safe value from what I know. Also it is good to have more space around taller components. Avoid having strange/not logical origins in your footprints. I always try to define the original either in the centre of the components, or at the centre of pin 1 if it is a component with very strange/none symmetrical shape. It might help the assemblers in some cases if you have a marker for the origin in your footprints.

    In general it is recommended to keep the angle between any 2 copper primitives > 60 deg (if you use Altium it has a special rule for that). I don't think it will be a serious issue nowadays, but small angles might not get properly etched during manufacturing.

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    • #3
      I would like to add to the excellent mairomaster answer:
      Regarding angel by which trace enters pad, is there any recommendation or limit which may affect assembly or test?
      - If you are asking from assembly point of view, I read some articles on the topic (they explain, that the way how pads will cool down may influence how components will be moved / not moved on the pads), then my answer is, that we had many designs where we only had certain ("not optimal") options to connect pads and we never have got complains from assembly house, so I would say there are probably ways how to handle it (e.g. controlled cool down process of the board (?)). Normally, you can ask for a feedback from assembly house after the first run and they usually have some notes and tell you what you can improve. It is very different between boards.
      - Also, according pick & place, many manufacturers do not like if you place SMT components too close to through hole components (e.g. close to through hole connectors or between pins of through hole connectors)

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