Ryan. again the best approach !
I have seen BGA's without voids on circuits with microvias, as you said,
when the hole was overplated, or after replacement when the operator
succeeded to fill the holes with solder during pad preparation, but even
then, there were some.
Gaby
----- Original Message -----
From: "Ryan Grant" <[log in to unmask]>
To: <[log in to unmask]>
Sent: Friday, July 11, 2003 10:46 PM
Subject: Re: [TN] BGA Voids
> Hi Joe,
>
> I hesitate to say micro vias in pad guarantee voiding, but... For most
> solder pastes, the flux vehicle becomes liquid during the flux
> activation stage of reflow. The liquid will wet where it can, AND also
> fill up the "trough" a via naturally makes, before the solder becomes
> liquid and can fill the trough. Driving the flux sideways out of the
> trough is impossible, and there is now a lot of flux to force by weight
> of liquid solder, to the top of the pad where it has a chance of
> escaping the joint. However, remember that "hydraulics" can support a
> lot of weight, so when the solder becomes liquid over the top of the
> liquid flux, it becomes semi trapped in the joint. At this point, it
> acts much like the shock absorber in your car. In order for the piston
> to move, the fluid in the shock absorber must exchange from one side of
> the piston to the other side. (otherwise you have hydraulic lock). In
> the same way, the liquid solder must "exchange" from on top of the flux
> to the bottom while it is in the cylinder of the via. Getting this
> exchange to happen BEFORE the BGA ball becomes liquid
> is...well...difficult. I won't say its impossible to achieve void free
> joints with micro vias in pad, but I've never seen one.
>
> However, if the via is filled and plated over so you have a flat pad,
> then of course it would be no different than a normal pad.
>
> Ryan Grant
>
> -----Original Message-----
> From: Macko, Joe @ IEC [mailto:[log in to unmask]]
> Sent: Friday, July 11, 2003 1:50 PM
> To: [log in to unmask]
> Subject: Re: [TN] BGA Voids
>
>
> Mr. Ryan,
>
> Great explanation of void formation.
>
> Does blind via's in pad always guarantee some voiding from your
> experience?
>
> joe
>
> -----Original Message-----
> From: Ryan Grant [mailto:[log in to unmask]]
> Sent: Thursday, July 10, 2003 5:07 PM
> To: [log in to unmask]
> Subject: Re: [TN] BGA Viods
>
>
> Hi Carl,
>
> I'm a little late in replying to your question, but anyways... BGA voids
> occur because the flux vehicle becomes trapped in the liquid solder.
> Remember of course that the flux vehicle is intermixed with the solder
> powder, and when the powder becomes liquid, it does not become liquid
> all at the same time. In fact, it may take several seconds for a paste
> deposit to become fully liquid. In the time between the powder changing
> from solid to liquid, the flux must be driven from the center of the
> paste deposit to the outside. Since the liquid will fall to the bottom
> of the paste deposit, and it is more dense than the flux, the flux will
> rise above the liquid solder. Surface tension will try to minimize the
> surface area of the solder that is liquid, helping to close any
> "tunnels" of flux that may exist within the liquid and drive the flux to
> the outside of the paste deposit. However, if the tunnel becomes
> closed, there will not be any forces to "push" the flux (and now the
> trapped volatiles) in any particular direction except straight up
> towards the BGA ball. This is the reason "flux only" BGA processing
> never has voids.
>
> As you can imagine, there are several factors that can help or hinder
> the entrapment of flux. First is the viscosity of the "liquid" flux in
> the reflow oven. (The viscosity of the flux during the dwell time has
> nothing to do with the viscosity of the paste during printing.) Reflow
> profile can certainly have an affect with the viscosity of the flux in
> the oven, however, each flux vehicle has its own unique way of
> responding. Some will gum up the longer it is heated, some will
> dissipate a little more, but most stay pretty consistent. This makes
> some solder pastes worse at voiding than others. Also, keep in mind
> that for many flux vehicles, as the flux removes oxidation, it turns
> into a gummy sludge. This is why nitrogen helps reduce voids; less
> oxidation to remove, less gummy flux residue.
>
> Another factor is how fast your spike to reflow occurs. As the solder
> powder changes from solid to liquid, it draws heat away from the
> surrounding powder, requiring additional heat to be input into the
> system. The faster you put heat into the system, the faster the solder
> deposit can go completely liquid. However, this also allows less time
> for the flux to escape to the outside of the solder deposit. Of course,
> once the flux is entrapped, it may still volatize and cause a small void
> to become large. This is why extended dwell above reflow has been known
> to increase the size of the void, and move them from being positioned at
> the PCB pad level to the BGA side.
>
> Another factor is the distance the flux must travel to get out of the
> solder deposit. Yet another factor is the volume of flux that must be
> displaced. Those two factors are the "size of pad and amount of solder
> paste" mentioned by your colleagues.
>
> Honestly, reflow profiles and paste deposit sizes are minor in
> controlling voids. The biggest impact on voids is the flux vehicle
> itself. If you have the ability to change solder paste models, that is
> where you will find the biggest change in void size.
>
> Ryan Grant
>
> -----Original Message-----
> From: Carlr Ray [mailto:[log in to unmask]]
> Sent: Wednesday, July 02, 2003 9:19 AM
> To: [log in to unmask]
> Subject: [TN] BGA Viods
>
>
> Hello All,
> I have a question (more like a poll I guess), concerning voiding of
> BGA devices.
> I have PBGA 480 IO paremeter grid device and I am seeing lots of
> voiding. Per the x-ray I am well within the IPC spec of 35% but I am
> still having failures at ICT (OPENS). Once I reviewed the profile I
> noticed we were not hitting the dewell time for flux activiation
> recommended by the solder paste vender (min 60 seconds) and we were
> seeing an extended time above 183c. We have changed the profile but some
> of my companions feel that the size of the pad and amount of solder
> paste is effecting the voiding. I disagree with them and feel that not
> activiating the fluxes and extended time above liquidous is a major
> contibutor to the voids.
> So now my question, what impact does the pads have with the voiding of
> the spheres on the BGA?
>
> Carl Ray
> Sr. Manufacturing Engineer
> Huntsville, AL 35807
> Phone: 256-882-4800 ext. 8845
> Cell: 256-990-1990
>
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