EEVblog Electronics Community Forum
Electronics => Projects, Designs, and Technical Stuff => Topic started by: george graves on April 06, 2014, 09:05:07 am
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Has anyone done this? How did it work out?
I have two designs, and one can use some cheap 0805's passive, the other might need 1206's. Other wise they are the same layout. So I was thinking of ordering/etching the same board with both foot prints on top of each other.
Bad idea?
(https://dl.dropboxusercontent.com/u/2295566/temp/0805and1206_footprint.jpg)
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how would the larger amount of solder paste on the pad effect the soldering of the smaller component?
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I would simply make the pads close together so the 0805 can span them. If you don't need paste and automatic placement, you can probably sneak one onto a normal 1206 footprint. (A 1206 probably completely covers an 0805 so the opposite wouldn't be such a good idea.)
Tim
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If you are doing this for a production/volume board, I wouldn't risk it. Of course if this is production, then talk to your assembly house and ask for their thoughts.
The problem with that much solder paste is that depending on how the paste starts to melt, it can cause the 0805 part to tombstone or move off the one pad. It's all about the surface tension as the paste melts.
I would suggest having 2 independent footprints (not merged) and then stuff one part or the other as needed.
Of course if you are hand assembling, then it's tiny volume and you can do whatever you want...
cheers,
george.
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I agree with Georges, for production it's a bad idea. However, if you're hand soldering, or only planning on making a few prototypes, you should be just fine combining them, or just making the 1206 pads bigger.
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And I thought I was the only one that did this...
I've made a 'component' in EAGLE that I use for this purpose alone.....barely enough of a gap between the lands for an 0402, barely wide and long enough to fit a 2512.
Sure, it's not optimal, nor would I use it in anything that would go out the door. Sure does make it easy to populate a prototype PCB with various stuff from the parts bins.
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Agreed. For hand placement it would most likely be fine. But I would not re flow in quantity like that. I am thinking if one side flows first capillary action may draw it too far away by the time the other side flows. Also the thermal consequences of having that much pad on a small component. May make cold joints.
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This sounds like something a cheapest Chinese manufacturer would do to allow them to buy whatever parts are cheapest at the Shenzhen electronics market that week to save a half of a tenth of a penny on cost. :-*
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Agreed. For hand placement it would most likely be fine. But I would not re flow in quantity like that. I am thinking if one side flows first capillary action may draw it too far away by the time the other side flows. Also the thermal consequences of having that much pad on a small component. May make cold joints.
Yep, and I run into that once in awhile, maybe a few out of a hundred SMT passives end up tombstoned and/or one end is cold.
Sure, takes a bit of time to hand solder those back down. In my case, it's quicker to grab a the first SMT 10K resistor out of the bin than it is to search for a 1206 10K resistor (or whatever).
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how would the larger amount of solder paste on the pad effect the soldering of the smaller component?
This is the primary problem, I have boards in production with 1210/1206 parts on pads with 0 toe, but the solder masks would need adjusting if an 0805 were to be dropped on these pads (which fit both 0805 and 1210 ok for hand placement).
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Why would you want to combine the two footprints? The 0805 is much smaller so you would be wasting valuable board space.
I use 0805 for prototyping these days anyway. Only time I use 1206 is for higher power applications like LED droppers.
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Those things work, but you need to experiment with stencil opening, soldermask opening and pad shapes and dimensions. This is perfectly doable in series production and after some tweaking we do not see any impact on yield. It's even doable for 3 or more components at the same time as well as less standard things (eg. power inductors).
I can't show you exact patterns, but believe me - this is a perfectly doable thing.
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Why would you want to combine the two footprints? The 0805 is much smaller so you would be wasting valuable board space.
Not disagreeing with that at all. The issue, if you want to call it that, is searching for a particular part in the bins. In my case, too many parts of too many different sizes in too many bins. Reach for the 10K SMT bin, grab one and go. And again, only for prototyping. Obviously wouldn't do this on a production board.
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Skimask
I have three SMD boxes. One for 0603, another for 0805 resistors and yet another for 0805 capacitors. And I'll probably have to get another some time for 0603 capacitors.
It's a pain but at least one doesn't have to have huge drawers like you do for leaded components.
I hardly do any development on leaded stuff anymore.