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I am uncertain as to the manufacturing specifications that I will need for a PCB I am hoping to get produced. The most complex component is the MCU (STM32151RCT) which has a recommended footprint for the LQFP64 ordering option as shown in the figure below: PCB Footprint Reccomendation

Most of the online PCB manufacturers that I have compared have ordering options for "Min Tracking/Spacing" of 4/4mil (~0.1016mm), 5/5mil(~0.127mm) or 6/6mil(0.152mm).

The recommended gap between the pads is 0.2mm which is wider than all of the above Min spacing options. Does this mean I can use the lowest tolerance 6/6mil spacing? Are there any rule of thumb design rules that are appropriate to this issue?

I have also referred to the discussion here Trace width and spacing, which suggests that it is best to avoid using the minimum size available due to quality issues that can arise.

Please also bear in mind that the cost increase in going from 6/6mil to 4/4mil is quite large, so if possible the wider spacing would be preferred.

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Any decent PCB manufacturer will be able to meet their published specifications. Will the yield rate be higher if you're not pushing the limits? Probably, but if you have the manufacturer test the boards, they're responsible for making sure all the connections are good, so they're not going to claim specs they can't meet.

Anecdotally, I've gone to 0.175 mm spacing on a 6mil/6mil spec without issue (for an accelerometer with 0.4 mm pin pitch), so 0.2 mm is nothing to worry about. If you want to be extra sure, the datasheet shows the maximum pin width as 0.27 mm, so you could take the pad width down to, say, 0.28 mm, which I've done when I needed to meet 8mil/8mil specs.

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    \$\begingroup\$ of course, taking the pad width down can have downsides – less easy to align pins on these, for example. I found that specific downside is often outweighed by it being easier to avoid solder bridges between pins if the pads are spaced slightly further apart. \$\endgroup\$ – Marcus Müller Apr 6 '17 at 9:01
  • \$\begingroup\$ Abe Karplus and @Marcus Muller - thank you both for your advice. Very much appreciated. I guess my initial thought was that there would need to be room for some multiple of the minimum track space - i.e 2* the 4/4mil spacing to equal the 0.2mm gap. Evidently it seems that I should be OK so long as the min space spec is less than the width of my smallest gap. \$\endgroup\$ – R. Fitz Apr 6 '17 at 13:21

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