I am sorry, Russ, but your EMS suppliers are right.
Additional volume of solder does NOT, and I repeat, does NOT increase BGA solder joint reliability for BGAs whose balls are intended to collapse during reflow (those with Sn63 and SAC alloy balls, but not Pb90). A larger cheese loaf is just that, a larger cheese loaf. It does not increase the modulus of elasticity.
The use of paste typically only increases the overall volume of solder about 1% or 2%, anyway. There is no real reliability difference in a solder ball that is .030" in diameter as opposed to one that is .031" in diameter, provided both are fully wetted on the pads.
IPC 7095 states:
"7.1.1.3 Importance of Paste Volume
For plastic BGAs much of their solder volume is supplied by the solder ball
on the part itself and the paste volume is not all that critical.
For BGAs above 0.80 mm pitch, stencil thickness will
be dictated by the other component types used on the
printed board assembly. Solder volume and stencil thickness
become more critical for ceramic and fine-pitch BGA
such as CSP. The solder balls used on ceramic BGAs are
not eutectic and do not collapse during the reflow process
(see Figure 7-1).
Because the high lead content ball does not collapse having
sufficient solder paste is critical. The fillet between the
land and ball depend upon the solder paste volume.
Ceramic BGA requires a minimum of 0.08 cubic mm and
a nominal 0.12 cubic mm paste volume."
Therefore, a requirement that solder paste be used for standard BGA rework is only going to lead to the following:
1. More Head-in-pillow solder joints caused by marginal reflow profiles, where the paste and the ball do not both go into liquidus.
2. More voids in the BGA solder joints. Solder paste is known to induce voiding in BGAs. Never mind what they say about BGA solder joints with voids not being a reliability issue; they are.
3. More non-agglomerated (renegade) solder fines and particles. It is difficult to manually screen paste and get it all on the pad.
4. More solder bridges caused by excessive or smeared paste prints, leading to additional subsequent reworks.
5. More flux residue. The flux in reflowed solder paste is much harder to clean than the tacky flux designed for rework.
6. Much more cost, from a rework standpoint. Applying a thin veneer of tacky flux on the board pads as opposed to carefully aligning a metal microstencil or stick-on stencil and printing paste is much less time consuming.
Solder paste is printed prior to BGA placement only because it is a convenient method of applying flux to the BGA pads during original SMT assembly. The additional metal volume (from the solder paste) is absolutely not needed.
If you are really concerned about BGA ball joint reliability, then increase the modulus of elasticity by providing solder columns rather than cheese loafs. This concept was explained in Werner Engelmaier's article awhile back, which included an outline of my cornerbonding process and my pictures of cornerbonded BGAs with solder columns rather than cheese loaves. Once this is done, the BGA typically ran several thousand more cycles before failure. The solder columns are not "added", they are created using the solder balls that come with the BGA.
I have reworked hundreds of thousands of BGAs without using paste, for many companies. None have shown any difference in reliability than BGAs either reflowed with paste as part of the original process, or reworked with paste. In fact, I have data that shows a reworked BGA using only tacky flux is more reliable. Why? Because a reworked BGA will have the benefit of pre-tinned pads with tacky flux applied, without any of number 1-6 above.
One final note: There should be little or no no-clean tacky flux seen after rework. A very thin "veneer" of a good tacky flux applied to the board pads is all that is required. I prefer water soluble tacky fluxes, and I am not afraid to state the Alpha WS 619 is an excellent halide-free tacky flux to use, with excellent cleanability in water.
-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Russ
Sent: Tuesday, July 05, 2011 10:15 AM
To: [log in to unmask]
Subject: [TN] BGA Repair Methods
Hello TechNetters,
I have a question about acceptable BGA repair methods. I have always required my EMS's to pre-apply solder paste to either the PWB or device prior to placing and reflowing a BGA. Some of my EMS's pushed back initially but always complied. IPC-7711/21B has two defined repair methods which both require adding solder by either pre-applying solder by using solder wire (section 5.7.1) or using solder paste (Section 5.7.2). No where does IPC-7711/21B state that you can get by with only applying tacky flux. Here is where I feel there is a conflict. In IPC-7095B Section 6.1.2 it states a couple places that you must "new solder paste or flux". No where does it mention pre-applying wire solder before applying the flux. IPC may want to review the wording there.
I am not sure how you would control manually applying core (wire) solder so I only consider pre-applying solder paste an acceptable method. I am a firm believer that by not applying solder paste reduces the solder volume, joint height and thereby reducing reliability of the solder joint.
First, I would like to know if anyone knows of a published reliability study of Flux only versus Pre-pasting a BGA repair.
Second, I would like to hear other opinions on this subject.
From a processing standpoint I understand the EMS's position that applying tacky flux makes the BGA repair a whole lot easier and cheaper as adding paste requires a mini-stencil and a trained operator. I am all for easy if it does not but reliability at risk. I also have a concern about the excessive flux that I find under a BGA that has been attached with only tacky flux. They are no-clean fluxes but but excess is never good or pretty. Thanks in advance.
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