BGA problem: open after reflow

Thomas: Wheeee, a 40% failure rate. Now, that make your boss chase you around the room won�t it?

As you indicated, x-ray won�t do dip about helping to identifying problems [or ball cracks either]. Consider using an ERSA scope [er whatever they call it] to inspect for your problem.

Dason is giving it to you straight. If your total gold thickness is 1 thou, you are getting too little gold. And this shortage will almost certainly give you a solderability problem, sooner or later.

A typical ENIG thickness specification spec is: 3-8 uinches of gold over 150-250 uinches of nickel.

The two counter-balancing issues with gold thickness are: 1 Gold too thin allows the nickel to corrode, making it very difficult to solder. 2 At below 0.5% of gold in solder no difficulties in soldering are encountered, but at higher concentrations, solder becomes sluggish when cooling. Concentrations of greater than 4% gold in alloyed solder, under plastic deformation, exhibit brittle fracture.

Finally [while not strictly on topic, but minutia close to being on topic], immersion gold self limits around 12 uinch.

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>We find that the raw BGA's pad sheared surface has the SnPb solder at various Z heights whereas the assembled BGAs pad leave a smooth layer of SnPb along the pad. Like to understand the cause for the difference.

Who could guess? Solder joint strength in pull/shear varies with lead geometry, solder volume, lead metal/metallization, and the way the test is done. It is very difficult to shear solder constantly. Shear tests are [in my opinion] senseless. The shear stress you measure depends more on the shear rate and on the point where the force is applied than on the grain structure. When shearing a component, you not only apply shear stress, but also roll the component over. This means that both shear and normal stress are present, again making the results very dependent on where you apply the load.

What�s the composition of the materials on the two solderable surfaces? Are they different? You mentioned gold. Are these both ENIG coated pads? Is your solder near-eutectic?

Without having a clue of what you�re doing [or how you�re doing it], I�d guess the rough surface was soldered well and the smooth surface was not. Do you get the same results popping the ball off with a screw driver [or torquing it off with a pair of pliers]? For me, this is the true value of shear testing � "bump pull" tests and the like only confirm quality, but do not assure reliability.

Can you post real nice pictures on-line on driveway, idrive, or someplace like that?

>1. During reflow, when the ball melts, does the ball separate from the BGA?

Which reflow? What kind of balls? What pads? What pad solderability preservative? What solder? Hel-low-oh!!!

When soldering the balls of a BGA to the assembly, I expect the ball to remain wetted to the [nickel plate over???] the copper pad on the BGA interposer. With a PBGA, like yours, I expect the ball will melt, collapse, and wet to the solderable surface on the assembly PCB. If you find the balls separating from the BGA, something is wrong, either with the BGA fabrication process or your material selection / process control.

> If yes, where does intermetallic layer stay? In the molten ball or the pad?

Intermetallic, which intermetallic? The gold should be dissolved in the ball and will form gold / tin intermetallic[s] where ever it chooses. Nickel / tin intermetallics should form along the boundary between the pad and the solder. Similarly if copper pads are involved, copper / tin intermetallics should form along the boundary between the pad and the solder.

>2. Can the flux from the solder paste flow to the BGA pads during reflow?

You bet, I�d be surprised if the solder paste flux did not flow to the BGA pads during reflow.

>3. Does this flux play a role in "remaking" solder joint made between the BGA's substrate and solder ball?

Yes, the solder connection at the BGA interposer and the ball will reflow, assuming the paste and solder ball alloys, and the solder on the BGA have similar liquidous temperatures, reach that temperature, and are solderable. While not quite the same, we have talked previously on this Forum about vias next to the BGA pads getting hot enough during wave soldering to reflow balls of a BGA located on the primary side. Cool [hot], eh?

>4. Does this SMT reflow degrade the shear strength of the joint between the BGA ball & BGA laminate after SMT reflow?

Listen, the pad will give it up before a properly designed and formed solder connection.

>5. What is the metallurgical characteristics of the NiSn Intermetallics?

The appearance and composition if the intermetallic layer depends on the temperature of formation. The brittle Ni3Sn4 forms on the nickel side. It�s tough to say what forms on the tin side. Some people say NiSn3, others say Ni3Sn2. I�m not taking any bets.

>What is the melting point / eutectic point of a NiSn alloy?

http://cyberbuzz.gatech.edu/asm_tms/phase_diagrams/pd/ni_sn.jpg

>During the SAM analysis we don't find the gold in both cases. We assume that the gold had migrated to the solder.

SAM??? You use SAM for metallurgical analysis? We use it to find package leaks and cracked balls. Please tell us more about this analysis technique. Who makes the machine are you using?

Anywho. Yes, gold dissolves into solder very quickly. You should be able to find the more common Au3Sn intermetallic with routine metallographic analysis.

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