Solder paste stencil - is this normal? - Page 1
Forum section: electronics / projects

author: zapta
author_rank: Super Contributor
timestamp: May 27, 2014, 07:21:07 pm
content: I order a stencil from a new vendor and the stencil I got is different from what I am used to.The vendor made changes to the stencil gerber design, adding little notches to the 1206 pads and changed the large TO-263 tab hole to a grid (on the right). The stencil worked well so I am not complaining but my questions are1. Is it normal for stencil vendors to do such post processing?2. What is the reason for these changes?3. Should I make these changes in the source so it will apply also to stencil vendors that manufactures the design as is (e.g. oshstencils or pcbpool).

author: mamalala
author_rank: Unknown
timestamp: May 27, 2014, 09:15:02 pm
content: While the changes seem to make sense, they should not change the the data that a customer sends but instead assume that that person knows what he/she is doing and go with it.Why the changes make some sense, IMHO: The 1206 pads seem rather long, a lot of the paste would end up under a non-metalized part of the package then, reducing that seems a good idea. As for the TO package, having too much paste under such a large tab may casue problems during reflow, the part may "swim" away at the slightest vibration, or the excess solder may squeeze out at the sides if the part is heavy (at least that is what i have seen).Greetings,ChrisEdit: Another reason for the TO pad change could be for the appliaction of the paste. Applying the paste with a squeegee on a large area may cause the inner area of that rectangle to have too little paste. The squeegee has some flexibility, after all. The added grid may help to prevent that.

author: Dago
author_rank: Frequent Contributor
timestamp: May 28, 2014, 05:07:10 am
content: Quote from: mamalala on May 27, 2014, 09:15:02 pmI believe the TO pad has been changed to a grid to reduce the amount of paste for such a large surface area (usually all large pads get considerably less paste) and to make applying the paste easier. If it's just one large hole it is very hard to get the paste to "stick" and not just kinda scrape it out at the same time you apply it.And I agree the 1206 pads seem to be very big so I guess they've just reduced the amount of paste for the pads.

author: AndyC_772
author_rank: Super Contributor
timestamp: May 28, 2014, 06:29:32 am
content: I agree, the changes that have been made have been done for sensible reasons. They probably have a database of feedback and issues that have been encountered when using stencils in the past, and have made modifications which they believe will result in the best performance of the finished item.But: if they didn't ask you first, then you'd be well within your rights to send it straight back and insist they manufacture what you actually ordered.It's very common for contract manufacturers (ie. those doing assembly and reflow) to adapt stencils from the original artwork, in order to work best with their own process. This is OK, and IMHO this way of working is preferable to insisting that the PCB designer makes any requested changes to the paste mask gerber. The finished stencil may be specific to a particular CEM, but the PCB design (including the paste mask layer) shouldn't be.My PCB footprint library certainly isn't specific to the preferences of any one CEM.As a general rule, I like to subcontract out the entire PCB purchase and assembly process to one supplier. Let them order the PCBs from wherever they like, modify the stencil to suit their own process, order their own components, and use their own solder paste and reflow profile. If the finished board doesn't work, then they can't blame the one part of the process that they didn't control.

author: luky315
author_rank: Regular Contributor
timestamp: May 28, 2014, 08:05:40 am
content: Usually they have a certain set of "rules" for relations between certain dimensions. But they should contact you before changing anything, unless you told the stencil manufacturer to optimize the stencil for your solder paste / process etc. They are really good at this and it's always a good idea to get their feedback.