[log in to unmask] on 01/29/98 04:57:08 AM Please respond to [log in to unmask]; Please respond to [log in to unmask] To: [log in to unmask] cc: (bcc: Brad Kendall/Hella North America Inc.) Subject: Re: [TN] Use of Solder Mask Defined pads for BGA ? Hi, My problem with NSMD pads is the limitation on the pad size for 50 thou pitch components. I have decided on dimensions for pad / via combination as follows. 30 thou pad with solder resist window of 24 thou and via pad of 28 thou with 10 thou via and a trace of 28 thou joining. 30 thou pads are as large as I can allow for 50 thou pitch and the 3 thou tolerance on solder resist gives a 24 thou window. 24 thou might be fine if 1) eutectic solder spheres (collapsing) are used, 2) the spheres are around 23 to 30 thou diameter. The JEDEC standard for BGA (collapsable)spheres is 23 to 36 thou. This is an huge tolerance (anybody have any insight as to why JEDEC settled for such a wide range?), and it's hard to imagine 24 thou pads with the 36 thou spheres plus solder paste. (Although after collapsing it is going to change shape). Has anybody experience of these dimensions in practice? The other thing is, how narrow can you risk with the solder resist border on top of the pad? Can you use a 2 thou border giving a 26 thou window? Or even 1.5 thou border? Obviously with non-collapsing spheres you have to put down alot more solder paste. 30 to 32 thou aperture size is what I would recommend. If you have 24 thou windows obviously you lead into the reason for your posting initially. I would say that printing solder paste onto resist is undesirable. It is dependant on the flux activity as to whether it wicks back onto the joint/pad without leaving balls. It is also dependant on the process (profile) as to whether it leaves solder balls behind. Especially in the case of no-clean pastes. It may be that for some people no problems occur printing onto the resist, but for others who have a no-clean with a lower flux activity and a profile which is on the edge of it's process window, it may not be so good. Can anybody else share their NSMD BGA dimensions with us? Regards We use 26 thou pads with a 5 mil annular ring. I do not recall the via size but they are tented. Brad Kendall At 11:30 28/01/98 -0500, you wrote: > Reply to: RE>[TN] Use of Solder Mask Defined pads for BGA ? > >Hi Johannes, >There is some debate about Solder Mask Defined (SMD) pads for BGAs. Some >papers indicate that Non Solder Mask Defined (NSMD) pads increase the fatigue >life of the solder joint (SMD pads cause stress concentration point in solder >joint). However, it is interesting to note that many of the BGA component >suppliers actually use SMD pads on the BGA substrate. So, although you can >choose SMD vs NSMD for your board, the supplier dictates the substrate design >(typically SMD). > >To address your overprinting question (printing paste on solder mask), we have >been using overprinting for Ceramic BGAs successfully for the past couple of >years without problems. The paste will pull back to the solder joint. You >may want to verify on your particular assembly (X-sections, etc.). > >Hope this helps. >Charles Elliott >Newbridge Networks > >-------------------------------------- > >The pads of PBGA`s that we use are defined by solder mask on our >multilayer FR4 pcb`s >The diameter of stensil aperture is bigger than the pad diameter , so we >are actually printing >( no-clean ) paste on the soldermask. >Is this OK, or should it be avoided ? I presume that the adhesion between >stensil >aperture-paste is bigger than the other one between soldermask and paste, >resulting >to increased risk to solder balls, which could be fatal under PBGA ( pitch >.05" ) > >Stencil width is 0.15 mm ( 6 Mils ), laser cut >Stensil diameter .56 mm ( 0.22" ) >Pad diameter 0.72 mm ( 0.28" ) max >Solder Mask diameter 0.52 mm ( 0.20" ) > >Any opinions? > >Brs, Johannes > >################################################################ >TechNet E-Mail Forum provided as a free service by IPC using LISTSERV 1.8c >################################################################ >To subscribe/unsubscribe, send a message to [log in to unmask] with following text in the body: >To subscribe: SUBSCRIBE TechNet <your full name> >To unsubscribe: SIGNOFF TechNet >################################################################ >Please visit IPC web site (http://jefry.ipc.org/forum.htm) for additional information. >For the technical support contact Dmitriy Sklyar at [log in to unmask] or 847-509-9700 ext.311 >################################################################ > > > ################################################################ TechNet E-Mail Forum provided as a free service by IPC using LISTSERV 1.8c ################################################################ To subscribe/unsubscribe, send a message to [log in to unmask] with following text in the body: To subscribe: SUBSCRIBE TechNet <your full name> To unsubscribe: SIGNOFF TechNet ################################################################ Please visit IPC web site (http://jefry.ipc.org/forum.htm) for additional information. For the technical support contact Dmitriy Sklyar at [log in to unmask] or 847-509-9700 ext.311 ################################################################ ################################################################ TechNet E-Mail Forum provided as a free service by IPC using LISTSERV 1.8c ################################################################ To subscribe/unsubscribe, send a message to [log in to unmask] with following text in the body: To subscribe: SUBSCRIBE TechNet <your full name> To unsubscribe: SIGNOFF TechNet ################################################################ Please visit IPC web site (http://jefry.ipc.org/forum.htm) for additional information. For the technical support contact Dmitriy Sklyar at [log in to unmask] or 847-509-9700 ext.311 ################################################################