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0603's on 0805 pads

#3154

0603's on 0805 pads | 28 August, 2000

We're trying out 0603 packages on some 0805 footprints (at this stage only on some test boards) since our vendor tells us the 0805's are going to be getting harder to get, and more expensive to pay for.

I understand the IPC footprint for the two is the same with respect to the spacing (according to the pcb design guys, anyway), so hopefully there won't be any need to print off pad. The parts do seem to fit nicely, albeit they're a little dwarfed by the pads. Anyway, we're also experimenting with aperture shapes to reduce solder beading, and I'm a little concerned about using a home plate because of the combined affects of mediocre placement accuracy and the decrease in the mass of the parts. Is it likely that I will see massive tombstoning if I don't hit the pads dead center?

And yes, I did search the archives for this stuff. Are we the only ones being told the industry is going away from the 0805's??

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Earl Moon

#3155

Re: 0603's on 0805 pads | 28 August, 2000

Steve,

If interested in more vital input - contact me offline. It's not just the factors you indicate but those of reliability.

Earl Moon

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Earl Moon

#3156

Re: 0603's on 0805 pads | 28 August, 2000

Steve,

This is a good, well repeated, not identified by IPC yet issue. I, and some others in industry, for years have proven and recommended smaller pads for various devices. I, and some others in industry, use 0402 pads for 0603 device types. IPC offers guidelines, not standards, as evidenced by the latencey of 7095. No criticism intended of their work. They take time for exerts like you to do the work and inform - through them.

Simply, solder joints are best smaller. Simply, smaller pads and solder paste amounts allow this.

The reason relates primarily to long term reliability as survivabilty of components initially and over time through thermal excursions and those mechanical as shock and stress. It ends solder joint and device end cap cracking.

This all started, and ended, with ceramic chip carriers and other devices so constructed in the late 1970's and early '80's. It's a simple matter of surface TCE (X/Y axis expansion at about 17 ppm/C vs. 5 to 7 or so). It's just taken a time for time to repeat especially with smaller chip device types.

It becomes more critical the smaller you require. It is essential for you to investigate more what many of us have known for years. It just keeps going on.

Earl Moon

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#3157

Re: 0603's on 0805 pads | 29 August, 2000

Earl, I emailed you prior to seeing your very informative post. Let me know if you got it so I know if that's a valid email or not. Very interesting. We clearly have some thinking to do.....

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Earl Moon

#3158

Re: 0603's on 0805 pads | 29 August, 2000

Steve,

Didn't get it. Send only to pod@ix.netcom.com and please include your real email address for reasons previously stated and obvious to all. Look forward to talking with you about a very interesting and important subject.

Enjoy,

Earl

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