Land pattern for LSM9DS1?

[Bill Burgess]

Posted on June 03, 2017 at 01:44

In TN0018 'Surface mounting guidelines for MEMS sensors in an LGA package', on Page 5 it offers different guidance on land pad dimensions depending on whether the 'LGA pin spacing' is (a) greater than 0.2 mm or (b) equal to or less than 0.2 mm. If (a), it says oversize the land pads by 0.1 mm in both length and width; if (b), the land pads are not oversized.

QUESTION 1: Does the phrase 'LGA pin spacing' mean open space between pins or pin pitch? I don't think I've seen a part with 0.2 mm pitch, to say nothing of less than that; the smallest microBGA parts I've seen have a pitch more than 0.2 mm. Anyway, the LMS9DS1 has a pitch of 0.43 mm, but its open space between pins is exactly 0.2 mm. Do we go by pitch, and oversize the land pads by 0.1 mm in both length and width? Or do we go by open space between pins, and not oversize the land pads? Or...?

QUESTION 2: In this thread -- 

<LINK NO LONGER ACTIVE>

 -- Miroslav B refers to TN0018, and goes on to suggest oversizing the land-pad length by 0.1 mm but the land-pad width by only 0.05 mm. This contradicts the oversizing suggestion on Page 5 of TN0018. If the answer to QUESTION 1 is 'yes, we should oversize the land pads for the LSM9DS1', then should we follow TN0018 guidance and oversize the land pads by 0.1 mm in both dimensions? Or should we follow Miroslav B's recommendation for the LIS2HH12, and oversize only the length of the land pads by 0.1 mm, oversizing their width by only 0.05 mm?

Thanks!

[Miroslav BATEK · Accepted solution]

Posted on June 11, 2017 at 22:08

LGA pin spacing means open space between pins.

Concerning the oversizing the land-pad is not that simple. 

I you want to get the best possible quality and repeatability you have to discuss this with 

PCB manufacturer and assembly plant. If you plan a mass production you should make a test batch to validate your desing and reliability of manaufacturing process.

I can give you only my recommendation based on my knowledge and experiences, If the assembly will be done by machine, do not oversize the pads. If you will do the assembly by hand oversize the length

of the land pads by 0.1 mm and the 

width

by only 0.05 mm.

[Bill Burgess]

Posted on June 12, 2017 at 05:18

Thanks!  Indeed I'm aware of good communication with fab and assembly being critical to success of mass production.

For this family of parts -- MEMS sensors -- ST Micro documentation emphasizes careful PCB and stencil design to minimize measurement error and distortion.  This is not something the board or assembly vendors will have any ability to speak to; they are only concerned with successful assembly, not consistent measurement quality.

So I'm very grateful for your guidance!  Thanks again.