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Cheating at PCB layout...making footprints too small is OK?

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treez

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Hello,

Recently i lost a contract for a small PCB layout for a xenon flashlamp.

I lost the contract because a rival agency layed the PCB out using nominal footprint sizes for the SMT resistors and capacitors........i instead made the footprints as big as the maximum 0603, 0805, 1206 etc footprint, and then added an extra 0.2mm on each end, to allow for slightly mispositioned components....

...as a result of making the footprints bigger, i did not have enough room for a ferrite bead that was needed for filtering for a particular customer, so i lost the contract.

But surely what my competitor did was wrong......i mean, if a large tolerance end batch of resistors comes in, then the resistors will have their ends sitting off the pad, and on the solder resist, which will denigrate the solder joint.?

I recomended making the pcb bigger and using my ample size footprints...but they binned me and went with the competitor...... is this right?
 

[...as a result of making the footprints bigger, i did not have enough room for a ferrite bead that was needed for filtering for a particular customer, so i lost the contract.]

Who make circuit, you or they ?

This is soldered by hands or machine ?
 
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I've always just used the standard foot prints and never had a problem even when I make my own for components I've not got them for, suggesting to a customer make PCB bigger is never a good idea the main reason as you know the bigger the pcb the more they cost, Your rival most properly used that against you with bigger footprints / bigger pcb and laid it on thick just to win the contract.
 
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If there were valid reasons that voltages requires bigger parts with bigger footprint and acceptable standards for reflow DFM dictated more space needed, I could understand your concern, but if the extra space was for hand soldering, in your case it would appear to be excess margin and in the other case insufficient margin which may cost them in terms of reject repair costs.

In the end it is about making the least cost reliable product. Unless you can convince them the other design will be unreliable from marginal solder joints, that is , if they will listen, you may not have any rights to complain, if they found a more labor intensive solution that fits. Not all requirements are easily met. ( a consultant might have resolved your design choice for DFM, DFC contention. Design is all about tradeoffs. Better luck next time.)
 

tolerances can be in many ways kept under control by using the standard foot print itself it was unwise of your part to quote that as a reason for increasing the size of the pcb board
 

Yes size of PCB and whole device is crucial but not only important thing.

Treez didnt answer on my questions from post #2. They should make tests of assembled device and see potential problems in working envirnoment.
 
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the size cost factor is still unclear does larger pcb cost more

i am new so still need a little experience in pcb designing
 

treez said:
...but they binned me and went with the competitor...... is this right?
Click to expand...
Nevermind the footprints. It sounds like you left out a ferrite which they specifically asked for, because they needed it. Surely that's enough reason for you to lose the contract?
 

tpetar,

The circuit is to be made by machine.

We all know eg 1206 reistors have size tolerance, i would have thought its just comon sense that you always use the maximum size.....then add about 0.2mm each end just in case pick and place machines have a bad day, or assembly staff are a bit "skiddy" on the solder paste.
 

The circuit is to be made by machine.
Click to expand...

For circuit I mean on circuit like schematic design, PCB is other thing.


We all know eg 1206 reistors have size tolerance, i would have thought its just comon sense that you always use the maximum size.....then add about 0.2mm each end just in case pick and place machines have a bad day, or assembly staff are a bit "skiddy" on the solder paste.
Click to expand...

That machine have tolerance of error, and I'm sure that manufacturer can handle two three PCB like loss.


Who knows maybe currently accepted PCBs will show up as bad in real life.
 
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treez said:
tpetar,

The circuit is to be made by machine.

We all know eg 1206 reistors have size tolerance, i would have thought its just comon sense that you always use the maximum size.....then add about 0.2mm each end just in case pick and place machines have a bad day, or assembly staff are a bit "skiddy" on the solder paste.
Click to expand...

If a PnP machine has a "bad day" that is the manufacturer's problem as well as if the assembly staff gets skiddish. You should always use standard footprints as those always count in for parts tolerances.

Your bad is the enlarging of component pads. Bigger does not always corelate with better.
 
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Also using larger pads for SMT may increase the chance of tombstones.
 
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Zasto.........."standard" footprints for 1206 etc do not exist....you get the min, typ and max...there is no standard.....its up to us if we gamble and assume that "max" will never occur.
 

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This is a very confusing post. There certainly ARE guidelines for 1206 footprints. And as someone mentioned, it seems like you left a part off the board (ferrite). That sure sounds like the reason you lost, not bigger pad sizes. Bigger pads doesn't necessarily mean bigger overall board dimensions, but if your board was, in fact, bigger, that is an actual cost.

But what is REALLY confusing me is: Did you actually DO the layout and then lose the contract? Why would you do the work without a purchase order/contract?
 
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each device has its own foot print which satiates the tolerance value of the components

and so there is no need for it to be made larger
 
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What did you use for 1206 outer pad pitch?
For reflow 150 thou (3.8mm) on 120 thou part allows 10 thou placement error. (0.25mm)
For wave 189 thou (4.8mm) on 120 thou part allows bigger exposed pad.

Perhaps customer need PCB to fit in a std box and ferrite was non-negotiable.
 
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Dear treez,

I can understand How you feel. as a customer point of view, one needs his design to be unchanged. cause this goes through lot of rules check while layout is being made. I wont say that your changes would spoil the design. but it might. an engineer gives a lot of guide lines to the pcb designer and also at the end reviews it. the layout constrains might be critical. when the product fails its the design engineer who is pointed out. most of the times I too have experienced the PCB mfg makes changes on their own, without the knowledge of the engineer.

Its good practice to get the changes approved by the engineer before its implemented.

Don't worry friend there can always be better tomorrow. and a failure is a cost we pay for earning experience.
 

Look at IPC-7351 THE ANSWERS ARE ALL IN THIS STANDARD
ALSO SEARCH FOR "THE CAD LIBARAY OF THE FUTURE"
These are standard footprints, the nominal size will cover most requirements.
 

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