Wide SOIC8 on narrow SO8 footprint - Page 1
Forum section: electronics / general

author: Faringdon
author_rank: Super Contributor
timestamp: November 07, 2022, 06:33:59 pm
content: Hi,We have a narrow SO8 footprint (UC2825). The part is wide SOIC8....please confirm we cant bend the legs to fit it just for the prototype?There is no room for changing the footprint.....if there was like a platform with a narrow so8 footprint, then we could maybe put the wide soic8 on that?...but do you know if such exists?UC2825https://www.ti.com/lit/ds/symlink/uc2825a-ep.pdf?HQS=dis-dk-null-digikeymode-dsf-pf-null-wwe&ts=1667845762740&ref_url=https%253A%252F%252Fwww.ti.com%252Fgeneral%252Fdocs%252Fsuppproductinfo.tsp%253FdistId%253D10%2526gotoUrl%253Dhttps%253A%252F%252Fwww.ti.com%252Flit%252Fgpn%252Fuc2825a-ep

author: wraper
author_rank: Unknown
timestamp: November 07, 2022, 06:37:46 pm
content: For prototype you can do a lot of things you won't consider for production. And bending the legs is not unusual at all.QuoteIf there is no room for changing footprint, where are you supposed to stick that "platform". You can do it yourself, just order small PCBs and solder on top of main PCB in LGA style. But it's very silly for production and not needed for prototype.

author: Faringdon
author_rank: Super Contributor
timestamp: November 07, 2022, 07:36:28 pm
content: Thanks, theres no room to change footprint, due to adjacent chip res and caps...but if the platform goes above these it will be ok......do you suggest like a castellated pad PCB, which solders to the too_narrow paads....which acts as a platform for another pcb with the wide footprint on it?

author: Kleinstein
author_rank: Super Contributor
timestamp: November 07, 2022, 07:40:57 pm
content: One can bend the pins to kind of make it fit. It depends one the actual footprint details on the PCB if and how good one can solder this. I have done that once with manual soldering (ordered wrong part and did not want to order again for just 2 chips). It worked, but it was not easy to solder and several cycles of rework. If the PCB is not yet ordered one could try to make the footprint a bid wider at least - every mm more helps.

author: DavidAlfa
author_rank: Super Contributor
timestamp: November 07, 2022, 08:23:29 pm
content: Do you mean this mistake...?You can, sort of, bend the pins and solder it like a PLCC:I hope it's not 2000 boards!

author: Faringdon
author_rank: Super Contributor
timestamp: November 08, 2022, 07:11:24 am
content: Thnaks yes, theyve said we should do it with folded legs, and then xray it to see if its soldered well.But wont the xrays kill the chip?

author: SMTech
author_rank: Frequent Contributor
timestamp: November 08, 2022, 08:36:48 am
content: I see this mistake all the time and do the trick above. Set the chip down on a hard flat surface and roll the legs under like an old fashioned J leg. If you do it firmly and evenly it works quite well. After that fit them to your pasted board reflow and make sure whoever 2nd loads, inspects and reworks as needed. If you could tweak the stencil to put down a little extra solder paste, even better. I don't see the need for x-ray unless the SO8 footprint was so mean as to be out of spec and no contact area was visible, but if you have it why not. I wouldn't be concerned about damage, X-ray is a standard inspection technique and is even used to count components on reels for stock control these days.

author: Psi
author_rank: Super Contributor
timestamp: November 08, 2022, 08:43:06 am
content: hehe, someone had to post the required BGA rats nest bodge fix.The things people do in development/RnD...

author: DavidAlfa
author_rank: Super Contributor
timestamp: November 08, 2022, 10:34:30 am
content: OMG, pain definition in a picture!Nah x-rays won't cause any harm, it's done all the time on complex boards

author: Infraviolet
author_rank: Super Contributor
timestamp: November 12, 2022, 06:42:38 pm
content: My avice, for the future in all your board designs would be, where space permits, have your SOIC and SO footprints with long enough pads than a thinner or wider chip can be easily soldered to them. Check the datasheets of an SOIC8 and SO8 part, use the minimum width between both datasheets for the distance between the inner ends of the pads and the maximum width acros both datasheets for how afr apart the outer pad ends should be. The pitch is the same so you might as well avoid trouble in future by being able to accomodate both chip types.