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Printed Circuit Board Assembly & PCB Design Forum

SMT electronics assembly manufacturing forum.

Can we hide extra long pad with solder mask ?

Denis C


Can we hide extra long pad with solder mask ? | 9 November, 2001

We are a contract manufacturer and we do business with a big company. There is a mistake in the design of the PCBs they are supplying: the pads are too long (under the component) and sometimes we get short circuit we cannot see. Changing the design with the customer is a long ride! Can we ask the board manufacturer to hide a section of the land with solder mask, we will then modify our printer stencil and get out of this problem ? Do you see any other problem we will get doing this ?

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Michael Parker


Can we hide extra long pad with solder mask ? | 9 November, 2001

If you are getting solder shorts under a device, how would "hiding" the extra long pad with solder mask help? If the pad can't be seen once the device is in place, it is already "hidden".

Getting your customer to redesign the pads may take some time as you say. You need a temporary solution. You are willing to redo the stencil. That is the right answer. BY maintaining a proper solder volume you will minimize or eliminate bridging under the device.

Design your apertures to have 10-20 mils of paste in front of the toe and behind the fillet. The paste will move towards the lead during reflow and not travel out all over the pad. Be sure to determine exactly where you will place the leads on the pads, measure the lead length and have the stencil aperture centered with respect to the lead center and not the pad center. By the way, you will probably have to get your customers permission to add extra mask before the fab house would make any changes. If the customer is willing to go that far, then they should go ahead and change the pad lengths to help solve your manufacturability problem.

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Can we hide extra long pad with solder mask ? | 9 November, 2001

Go for it!!!

You were unspecific, but adding mask to a completed board could cause: * Increased paste thickness, if you have double thickness of solder mask. * Wrinkled and lifted solder mask under the component, if the solderability protection of the portion of the pad that�s under the solder mask is HASL or imtin. * Solder mask adhesion problems to the gold surface when applying gold then solder mask, if the solderability protection is ENIG. Solder mask residues can also be left on the gold, resulting in solderability issues during assembly.

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