Solder mask bridge issue
From what I understand, PCB manufacturing has a limitation on the minimum width of the solder resist—it cannot be too thin, typically not less than 5 mil. In many components, including those with a 0.65 mm pitch, the spacing between pads can become smaller than 5 mil.
This reduction in spacing is due not only to the proximity of the pads but also to the solder mask opening—the larger it is (e.g., a common value is 4 mil), the smaller the remaining solder mask bridge. However, this issue persists even when the solder mask opening is as small as 2 mil.
For example, I am currently working with the NCP730BMT280TBG (LDO, see two images below) and the PMPB15XP (P-channel MOSFET, see last image below). Both of these components have tight pad spacing, making it difficult to maintain a sufficient solder mask bridge between them.
Is my understanding correct? If so, how is this issue typically addressed?
Additionally, I'd like to clarify terminology: Does "SM bridge" mean the same as "SM sliver," and is this the issue I am describing here?
What are the recommended design practices for professional manufacturing (without strict compliance to specific standards)? What is commonly accepted in various industries?
Thanks in advanced to any answer,
Eyal
2 answers
Talk to your board house. 4 mil slop for the soldermask around pads is excessive. Leaving 4 mils is fine for larger parts, but as you say, it doesn't work for fine pitch.
The main reason for making the soldermask hole larger than the pad is to allow some room for mis-registration between the copper and soldermask layers. Some error is inevitable, but 4 mil is excessive.
Another reason is to make sure the pins are touching the pad, not up off the pad a bit because there is a layer of soldermask under a corner or one edge of the pin. Usually you make the pads a little larger than the pins anyway, but for tight pitch you usually do 50/50 pad and space between pads.
Again, ask your board house what they suggest for what works well with their capabilities and process. If the answers sound wishy-washy, you may need a better board house.
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Thank you, @Olin, for your response!
Regarding the SM expansion around the pad - this is a good input, I wasn't sure about it.
Regarding SM sliver, I am working with PCBWay, and unfortunately, I haven't been able to fully understand their capabilities for the cheapest board option (which I assume is their "Standard" service). Their website seems somewhat unclear, and based on my correspondence with them, I’m still unsure.
From their website:
And here is the correspondence:
-What is the minimum solder mask sleeve/bridge (depending on color)?
-white, black, matte black, matte white, these 4 requires min 0.22mm IC spacing, all other colors can do with 0.19mm.
could it really be that large?
Perhaps someone with experience working with them can clarify.
In any case, my original question was about common industry practices, and I’d like to add:
What are the typical values for the minimum solder mask sliver/bridge in standard PCB manufacturing (nothing special or high-end)?
To refine my question further:
At what pitch does a design become too fine for standard manufacturing and require special fabrication processes?
Additionally, are there alternative solutions? I’ve come across the option of removing the solder mask between pins, but I assume this is not ideal.
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