Jack;

The composition of the members the IPC-SM-782 committee include designers,
fabricators and assemblers. Their experiences come from thousands of real
life designs and assemblies. The main advantage to producing a standard in
this manner, is that they are all willing to share their experiences to
further better the industry. The IPC paper document is for sale, but the
technical information remains in the public domain. 

The IPC standards receive a technical review from sources located around the
globe. These sources are users and designers of a broad spectrum of products
and applications. Yes, a standard, by committee, represents some
concessions, but none that sacrifices quality, reliability, or cost. When I
look at an industry standard, I use the information as a guide for what the
majority of the industry can accomplish in a cost effective manner. I may
wish to taylor the information for my particular process, but at least I'll
know what the long term effects will be.

At 12:32 PM 6/24/96 -0700, Jack Olson wrote:
>Fred Pescitelli wrote:
>> Has anybody been using SMT Plus land patterns? They are advertising a lib.
>> for P-Cad. How do they compare to the IPC 782 Patterns?
>> Jim Blankenhorn (of SMT PLUS) once wrote an article commenting on the 
>IPC-SM-782 document. It was common sense type notes concerning several of 
>the footprint dimensions in 782 that were questionable. I'm sure he would send 
>you a copy if you call him. Personally, I have never heard a complaint about 
>anything in the SMT Plus library, and I tend to trust someone who gives 
>solutions based on 1000's of real-world assemblies rather than the ones
decided 
>"by committee"
>
>> What are you doing as far as via's and conductors under SMD components? I
>> noticed the P-Cad lib has keepouts under the chip components. I have been
>> routing traces under SMD and have not had any problems.
>> It has been quite awhile since I've even SEEN a PCAD library part (we
make our 
>own) and am kinda surprised about built-in keepouts. Several recent articles 
>mention having to customize glue dispensing over traces in high volume 
>assemblies so it might be good to speak to the assembler about it. But look at 
>other boards; almost every design you see has traces under chip devices! From 
>my experience vias are okay if they are tented, otherwise the inability to 
>clean properly may cause early failure (from corrosion). 
>
>> Silk Screen legend sizes seem to be very large on most commercial lib's
>> compared to what I have been using. I seem to get away with .008 width and
>> sometimes down to .060 height. What is your experience?
>> For the last three companies I've worked at, 60high/8wide was typical, and 
>50high for tight placements. One consideration about the width is that 8 looks 
>crisp, very legible, but for higher volume the screen can get clogged, which 
>will start leaving gaps.   
>
>> I have just finished a Design for a client that demanded that all components
>> be orientated in the same direction and also all of them lined up, nice and
>> neat. Great idea if I would have had twice the board area to work with. I
>> know the desireability of this but I also like to think form follows
>> function. Many of the original reasons for doing this are obsolete. The
>> board is finished now but I just know placing the parts for optimal routing
>> would have resulted in much shorter runs, and a more functional layout. What
>> is your opinion? How are you doing it?
>>Tough question, because I sometimes can't help going for beauty if I have the 
>time, the artist in me I guess... but not if the circuit is degraded. 
>I think you might benefit from asking more questions up front just to 
>understand the issues your customer/company is considering. Maybe they want 
>your board to grace the cover of their new brochure, eh? 
>
>> I also was prohibited from putting chip caps on the solder side, even though
>> I routinely do it with other designs. Are any of you designing boards that
>> are waved with chip caps on the solder side?
>>Sometimes what would benefit you as a designer could cost the company a
lot in 
>the long run. Even if you only want to put a few components on the back, it 
>complicates the assembly drawing and may affect training feild service
manuals, 
>parts may not be accessible after installation, Assemblers typically charge 
>more automatically for double-sided boards and more steps are required, 
>depending on the volume it might not be worth programming a pick-and-place 
>machine for only a few parts on the backside which means those will have to be 
>hand assembled; there are any number of reasons for making design decisions.
>Again, you may want to start asking more questions...   
>good luck
>								Jack
>
>
Regards,

Gary Ferrari
Tech Circuits
(203)269-3311
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