Don’t’ expect a simple answer – that depends on your application.
If you are looking for a solder paste, NC-SMQ80 is the flux vehicle of choice. For spray applications 5RMA has a strong following on Au substrate applications. For niche ball attach or specialty processes needing a tacky flux, WS-363 delivers the best wetting on ENIG (see attached chart), but WS-575 has a more useable rheology for pin-transfer. PoP Flux 030B is your ticket in no-clean applications at these low temperatures.
Did I mention all these options are Halogen-Free?
I’m glad to mention a new application note has been added to our database. This recent addition deals with the topic of pin transfer. Follow this link and click on “Variables of Pin Transfer” to read more.
Following the success of NC-506 (No-Clean flux for mounting spheres on BGA packages) is a halogen-free version – NC-585. I’ve been working with this flux quite a bit lately to see how it stacks up with the industry leading NC-506.
The first thing I noticed was the lower tack and viscosity of NC-585. This was apparent in initial pin-transfer testing. The flux rolls well under the squeegee blade and transfers a large amount of flux per pin. Because the transfer is so pronounced, NC-585 may bridge between very closely spaced pins, we will need to take a closer look at this later. Normal BGA pitches are no problem at all with this flux.
The wetting of this material was surprising. I didn’t think any halogen-free flux would be able to compete with NC-506, but NC-585 had even better wetting characteristics on bare copper and ENIG surfaces at Pb-free temperatures with SAC 387.
The future of this flux looks promising, as does the future of powerful halogen-free fluxes.
I really do mean art… This image was generated by Michael Riddle of Cyber Technologies for a project we are working on. Maybe I’m just crazy about this kind of thing, but I wouldn’t mind having this hanging on a wall in my house – it’s just cool!
Mike let us in on how to create these very descriptive images:
“We call the process of separating deposited materials from the substrate “Surface Subtraction”. The steps are easy to follow:
1) Fixture and scan the bare substrate (Scan A)
2) Deposit the material to be measured
3) Re-fixture the printed substrate and re-scan, using the same settings as Scan A. We will call this scan “Scan B”.
4) In our software we can subtract one scan from another. We call this feature “User-defined Compensation” .Here, we subtract Scan A (the bare substrate) from Scan B (the printed substrate) leaving only the deposited/printed material.
For the 3D image, I first plotted the 3D surface map of the bare substrate, and then plotted the 3D surface map after the user-defined compensation was enabled.”
Data is the output that we initially look forward to, although tech presentations need some ‘spice’, and this type of creative tech art can really drive home the point you are trying to make with your data.
This is the shape of flux deposits left after pin transfer
Spheres attached after pin transfer
Pin transfer is a way of selectively depositing a semi-solid or liquid material (like a solder paste or ball-attach flux). It is commonly used to apply flux to BGA (ball-grid-array) pads to promote subsequent solder sphere attachment. Pins are dipped into a reservoir of material where the pins are coated with flux, paste, or epoxy. Next, the pins are lifted out of the material reservoir and placed down onto the BGA pads. A portion of material that traveled on the pins sticks to the pads as the pins are lifted away. As archaic as it sounds, this method is quite repeatable – and used extensively in semiconductor packaging. This method of application is used because it deposits flux very quickly, and can compensate for changes in substrate height.
Pin transfer fluxes are specially designed with rheological characteristics to help optimize the amount of material that is picked up and placed on the pads. Other materials can also be pin transferred. Pin blocks are used to transfer solder paste and epoxies in a range of applications. Sometimes only one large pin is needed to transmit flux, this is called flux stamping. Stamp transfer has the benefit of being simple to set up because there are few parameters that need to be adjusted.
"A picture is worth a thousand words." Reliability is worth even more…
Reliability is such an all-encompassing term, we had better start with a definition: In general, reliability (systemic def.) is the ability of a person or system to perform and maintain its functions in routine circumstances, as well as hostile or unexpected circumstances.
The IEEE defines it as ". . . the ability of a system or component to perform its required functions under stated conditions for a specified period of time." – Wikipedia
A natural question to ask is, "what is your measure of reliability, what is your failure criteria?" The failure modes which are common due to ball attachment pin transfer and sphere placement are missing solder spheres and poor solder joints caused by inadequate flux volume. Of course there are other BGA defects like voiding and excessive intermetallic growth – but those defects are related to reflow instead of flux / sphere application.
Let's work backwards. I would expect two issues to cause reliability concerns, 1) inadequate flux volume and 2) excessive flux volume. Not enough flux on the substrate pads will cause spheres to not solder correctly. Since flux battles oxide formation and promotes wetting, it needs to be on the pad and it needs to have enough body to perform its task. It could potentially make cleaning a water soluble flux more difficult and probably won't hold the spheres in place on certain high acceleration automated machines. The process variables that may cause inadequate flux volume are shallow flux reservoir depth, small pins, fast pin motions, and inaccurate dwell heights above the substrate or reservoir base. A large change in viscosity (lower) can also lead to an unexpectedly miniscule amount of flux being transferred.
Excessive flux isn't as common of a concern but it can impact BGA assembly by allowing spheres to shift out of alignment, contributing to thick and dark no-clean residues (in the no-clean process), or just plain causing a mess inside the sphere mounting equipment. Larger flux volumes are caused by the opposite of each process variable listed previously for inadequate flux deposition, as well as pin bridging – the entrapment of flux between transfer pins.
This is just an ultra low resolution picture of flip chip attachment flux, however ultra low residue flip chip attachment flux is clearly a great solution for no-clean applications which involve subsequent capillary underfilling (pun intended).
There are low residue fluxes designed for dipping, pin transfer, printing, spinning, or spraying. But low residue fluxes do have a unique limitation. Because the solids content of the flux is low and it offers less protection from oxygen at high temperatures, low residue fluxes generally need to be used in an inert atmosphere. And now the good news: most of us are already using <50ppm O2 in our flip chip processing furnaces anyway. It looks like in this case less really is more.
We commonly think of flux itself as the reason it should only be used for 8 hours at a time. As the workable life of new fluxes increase, we should also consider any effects the equipment might have. In recent work with extended working life ball attach fluxes, I have found a disturbing issue with certain pin transfer machines, a problem that is not noticeable under typical conditions.
The picture above shows some ball attach flux removed from a pin transfer flux reservoir after 72 hours of continual usage. The dark area is ball attach flux with imbedded aluminum particles from the doctor blade assembly. The friction during usage must not have been accounted for in this machine’s design. Under normal use (changing the flux after only 8 hours) this issue has not been seen, it becomes pronounced sometime during the second shift.
This additional strain on the equipment should be realized before the making the decision to extend the time between cleanings. I wouldn’t want BGA’s assembled with aluminum particles suspended in flux and I’m sure you feel the same way. The fix for this issue may be simply adjusting the squeegee pressure, or as involved as having a new reservoir / blade assembly machined out of different wear-resistant materials. That really depends on the level of adjustability your equipment has.
One very important part of materials testing is to find the usable limits of a material before it becomes a released product. Some customers like to use ball attach fluxes beyond the standard 8 hour recommendation – and in some cases this is possible.
I like to find the realistic limits of ball attach fluxes using our pin transfer unit. If flux has been left on equipment too long, the first sign of failure is generally bridging flux deposits that appear across the same pads print after print. Inspection of the pin transfer array should show a string of flux spanning the gap from pin to pin. The interesting thing about this phenomenon is that most ball attach fluxes in the industry have a threshold after which multiple bridges occur almost at once. This makes finding the ‘workable life’ of a flux much easier.
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