Printed Circuit Board Assembly & PCB Design SMT Electronics Assembly Manufacturing Forum

Printed Circuit Board Assembly & PCB Design Forum

SMT electronics assembly manufacturing forum.


Via-in-pad

Michael Allen

#7798

Via-in-pad | 17 January, 2000

I'm searching for technical papers on the topic of via-in-pad (through-vias, not micro-vias) -- hopefully with reliability test results.

There's been zero response to this question in the past, but I thought I'd ask again. References would be greatly appreciated!

Regards,

Michael Allen

reply »

#7799

Re: Via-in-pad | 18 January, 2000

Michael: Your only solace may be the anecdotal responses in the archives. Good luck. Dave F

reply »

Jeff Sanchez

#7800

Re: Via-in-pad | 22 January, 2000

Michael, I am not sure if you are trying to lay out a board with vias in the pads or suffer through an assembly you got stuck with? I am building run of boards right now that has vias in most the smt pads. I'm sure the design group thought it was a great place to hide the vias but I don't. Think of it this way. You print your paste on the board and run it through the oven and oops, where did all the solder go? Wow, we need thicker stencils? But what about the pads that have no vias? They will have more solder than the pads with vias. I suppose if you look at the standards for "pin to paste" processing it might allow for less solder on the pads that have vias. I am not sure if those standards are complete yet. Someone like Bob Willis might be good to ask. He is listed on this site at the book store under soldering. We are currently assembling this run by hand and charging for it. I told the customer if he wanted this board to ever be built in an automated process he would have to find a new home for all his vias or live with the lack of solder on the pads. It looks good to have all the vias tucked away but I don't think it's wise to hide them in the vias. If you must, at least make them as small as possible. Just a few thoughts..............Jeff Sanchez

reply »

#7801

Re: Via-in-pad | 23 January, 2000

Jeff: I feel your pain. Your points about pin-in-paste stuff are well taken. Consider increasing the size of the pad containing the via.

* Calculate volume of the finished via. * Multiply this volume by 2 to determine the volume of paste that must be added. * Divide this volume by the stencil thickness to determine the amount of area to increase the aperture.

You can over print pads by 10-15% without solder balling.

Good luck

Dave F

reply »

Mike Gamble

#7802

Re: Via-in-pad | 23 January, 2000

No insult intended but you are either brave or being bullied by design. My guess is the latter as no assembly plant really likes doing things against the grain of norms. We have done Via in Pad with reasonable success but it is fraught with hidden traps. Surprise #1: you dont need extra paste! Believe me Surprise #2: Reflow profiles need to be EXACT! Surprise #3: Make the via big enough to allow outgassing and DO NOT block the bottom of the hole

There is a new PCB material being used for VIP process, I dont have the name at hand but if you are interested I can get it for you. I believe that this is going to become an accepted industry standard on BGA but shudder to think of trying it on Micro-BGA!! I would be happy to answer any specific questions should you need help.

reply »

Jeff Sanchez

#7803

Re: Via-in-pad | 23 January, 2000

Hey Dave, If ya wanna feel my pain come help build these little babies......hehe. Thanks for the process. I will print your thread and give it a shot. It sounds good to me. I am not so sure if I can get them to inlarge the pads but I can try? The pads can stand to go a size or two larger. Thanks Dave.

Mike, That was good imput as well....Thanks

Jeff Sanchez

reply »

#7804

Re: Via-in-pad | 24 January, 2000

Jeff: Whoa bud. I'm sorry. Leave the pads as they are. The calculation was intended to increase the aperture for the pad with the via to compensate for the paste starving caused by the via. Sorry for not being clear. Dave F

reply »

Jeff Sanchez

#7805

Re: Via-in-pad | 24 January, 2000

Thanks Dave, I see it now..........Jeff

reply »

Michael Allen

#7806

Re: Via-in-pad | 25 January, 2000

Thanks for the input Mike (and others).

As you guessed, via-in-pad is not my idea, and I'm resisting it as much as I can. One of our designers used this practice at some other company, says there were no problems, and wants to use it (via-in-pad) as the default via placement scheme! Our first via-in-pad board had a gold finish, and the 0603s had essentially zero fillet -- it looked like there was no solder at all. A second via-in-pad board has a HASL finish, and the 0603 fillets are present but still much smaller than normal.

I've argued that this solder "drain" could affect reliability of the board, but I'm not finding much literature to back me up. We're considering doing our own reliability study, but I don't want to reinvent the wheel. So that's where I stand. If I find any papers on the subject, I'll let you know.

The answer might be to have the vias filled (with epoxy or ?) and plated over. I'm looking into that possibility as well.

--Michael

reply »

#7807

Re: Via-in-pad | 25 January, 2000

Michael: On reliabiliity, consider that:

� IPC-SM-782, "Surface Mount Design And Land Pattern Standard" and IPC-D-279, "Design Guidelines for Reliable Surface Mount Technology Printed Board Assemblies" specifically describe via in pads as "poor design practice." � IPC-A-610B, "Acceptability Of Electronic Assemblies" describes proper looking solder fillets for your class of equipment. � Properly done, the size of those globs of solder attached to the connectors of SMT components determine solder joint strength and reliability. - Within limits, solder connections with more solder tend to be more reliable or stronger than those with lesser volumes of solder. (SP Hawkins, CJ Thwaites, ME Warwick "The mechanical Properties Of Soldered Joints To Surface Mounted Devices," �Brazing & Soldering,� No 10, Spring 1986) - Land extension of less than 0.045 reduces reliability (C Capillo "How To Design Reliability Into Surface Mount Assemblies," �Electronic Packaging and Production,� July 1985) - Solder connection mass absorbs movement differentials in materials caused by differences in CTE of the board, pads, and component.

... and influence solder defects, cleanability, testability, and repair/rework.

Recognize there will be times that vias will be in pads due to reasonable design requirements (high frequency design - good reason)(we've always done it thatta way and its variants - obviously poor reasons). In those case, you need to deal with it. Mike's comments about profile, (as you say) plugging vias, and over printing are in your tool kit.

Good luck

Dave F

reply »

Electronics Equipment Consignment

SMT in-printer dispensing