Following on from our discussions of last time...
As you will recall from the previous post on this topic, My friend and colleague Chris Nash and I were discussing some puzzling results for low dip height found during testing of package-on-package (PoP) materials. The findings will be of interest to everyone who uses a dipping process in both SMT and flip-chip assembly.
As always, please contact me if you need to learn any more.
Cheers! Andy
Solder powder particle size and shape impacts the functionality of solder paste in many ways: printing/dispensing/dipping; solderballing; graping; voiding; tack and so on.
For this reason, I just spent an interesting couple of months leading a cross-industry (two solder paste suppliers and two solder paste users) group to help my old friend Brian Toleno, chair of the IPC 5-24b (Solder Paste Task Group) put the finishing touches to the final version of the J-STD-005A. The concerns were with the definitions of powder size in paste: both the distribution and the “maximum allowable particle size”. We reached a nice pan-industry consensus, which should allow the J-STD-005A to see the light of day as a published document in 2012. I also saw some recent work by colleagues on the effect of particle size on surface area. I didn’t see the derivation of this work, so I want to show you how to calculate the surface area of solder powder in a paste.
Assume solder paste at a weight loading of x%. [Note that: As the solder powder size (diameter) decreases, the metal loading is usually also decreased by 0.5% or more to compensate for the boundary layer of thixotropic flux adhering to the particle surface, but let's make the first order assumption that x is independent of particle size]. So 1 gram of solder paste contains (x/100) grams of solder metal.
If the metal has a density of r (rho), then the volume of metal (v) per gram of solder paste:
v = x / (r * 100)
Let’s assume that the metal particles are monodispersed (i.e.: all the same diameter (d)), so the number of particles per gram of paste (n) is then simply v (total volume of metal per gram) divided by the volume of one particle (vp).
n = v / vp = x / (r * 100 * (4/3) * pi * (d/2)3 )
We can now also calculate the solder powder surface area (s) per gram of paste from our knowledge of n and the surface area per solder powder particle (sp):
s = n * sp = n *4 * pi * (d/2)2
It is a simple matter of algebra to show that the ratio of surface area to volume is merely an inverse of the particle radius or diameter (I’ll leave that as homework for you):
| Metal loading = | 90 | 90 | 90 | 90 | % |
| Metal density = | 8.4 | 8.4 | 8.4 | 8.4 | g/cm3 |
| Powder particle diameter = | 60 | 40 | 20 | 10 | microns |
| v(p) = | 0.000107 | 0.000107 | 0.000107 | 0.000107 | m^3 |
| .: in 1 gram of paste, n = | 9.47E+08 | 3.20E+09 | 2.56E+10 | 2.05E+11 | particles |
| surface area = | 10.71 | 16.07 | 32.14 | 64.29 | m^2 |
A while back, I did a little Excel numerical integration to show the effect of powder type on the population distribution, and hence how powder “type” (2,3,4,5 and so on) affects the surface area, with some assumptions thrown in about the width of the distribution. The results are shown below, and are pretty much as you would expect. As you go from type 3 to type 6, you see about a 10 fold increase in the surface area.
This week a customer in Asia asked why one of our new epoxy fluxes was not allowing the package-on-package (PoP) device to be picked up from the dipping tray. Obviously, the vacuum nozzle must have sufficient force to extract the PoP package from the PoP flux reservoir (yellow, below).
Those of you who know me also know that I am always trying to reduce things to numbers so, naturally I got thinking about how I would model this from a physical viewpoint and came up with the following:
If the downward force (weight of component plus tack force of epoxy flux) is greater than the upward force (air pressure on the bottom of the component), then the component could not be extracted from the epoxy flux. The figure shows the different variables. Expressing this mathematically, this comes out, in SI units, as:
Downward force = m.g + n.Ft.pi.(d/2)^2
where Ft is the tack force in units of mass per unit area, taken from the maximum tack force determined by the Solder Paste Tack Test from J-STD-005, ANSI/IPC TM 650:2.4.44
Upward force = 101000.A.pi.(D/2)^2
where A is the measure (fraction) of atmospheric pressure and denotes how good the vacuum is (zero vacuum is 0.0atm : hard vacuum is 1.0atm).
There are some uncertainties with this approach: How does the vacuum vary across the nozzle diameter? Does the 5mm diameter probe used in the IPC test equate to a complex CSP (chip-scale package) bottom surface, with many rounded solder bumps or solderspheres? And so on. But, at least the model puts us in the right ballpark. Just to give you a feel for how this works, the second figure shows some results. Note that scenario (iv) is the only one showing problems (negative force balance).
The data implies that you are only likely to see an issue with inability to pick up components from a dipping flux tray if either:
For many of the newer applications, component sphere/bump immersion to just deeper than the bump height (say 100-110%) is desirable. If the customer dips the whole bottom of the component into a standard (non-epoxy) flux, this potentially opens up a lot of issues including reliability (SIR; electrochemical migration); component displacement (skewing) during reflow; as well as difficulty in picking up the component from the tray. The solution to this series of issues, is to choose either a standard flux with a high pre-reflow SIR, such as our PoPflux 30B, or a low-volatile content epoxy flux.
I'll have more to say on epoxy fluxes in a couple of months, as we are currently nearing the end of extensive testing at several customers in Europe and Asia.
上週在美國的拉斯韋加斯(Las Vegas), IPC舉辦了美國地區行業的盛會APEX. Indium公司一如既往的在展會中心安排展位，和業界各位舊友新友交流，與大家分享最新的產品和技術，傾聽大家的反饋和聲音。
除此，在人山人海的技術會議交流中心(paper presentation, educational workshop)，Indium公司的五位大將還為大家做了精彩的演講：
² Lead-Free Flux Technology and Influence on Cleaning.
² Selection of Dip Transfer Fluxes and Solder Pastes for PoP Assembly.
² Achieving High Reliability Low-Cost Lead-Free SAC Solder Joints Via Mn or Ce Doping.
² Achieving High Reliability for Lead-Free Solder Joints – Materials Consideration
² Addressing the Challenge of Head-in-Pillow Defects in Electronics Assembly.
² Challenges for Implementing a Halogen-Free Process
² Understanding SIR
² Stencil Printing Transfer Efficiency of Circular vs. Square Apertures with the Same Solder Paste
Cheers!
Click here for a description and video that shows a nozzle design from FINETECH which clamps down onto PoP components during rework.
“The PoP soldering head is an easy-to-use tool for reworking stacked devices as a whole in a single reflow process. It uses vacuum-actuated mechanical clamping tweezers which avoid separating the single layers of a PoP during component removal. The PoP soldering head can be easily adapted to different component thicknesses. Furthermore it is possible to adjust the width of the clamping tweezers prior to the process when the rework arm is swiveled down to avoid affecting other components on the PCB (e.g. accidental shifting of neighboring small passives).”
Sounds like this would be great for combating “PoP Quicksand”. That nasty problem that large components have when the vacuum provided by the nozzle isn’t strong enough to lift the package-on-package component back out of the PoP solder paste or dipping flux. Okay, I just made up that term – but it’s pretty descriptive, right?
Conceptually it seems to make a lot of sense, please comment if you have any experience with it!
Source: Amkor
The new TVM™ PoP components have gotten a lot of press, and soon we should be able to get our hands on TMV PoP daisy chain parts for material testing. I would love to evaluate next generation PoP solder pastes with these new components. Lee Smith (Vice President of Business Development at Amkor) had this to say about obtaining parts for testing:
LS: “We have significant direct and end customer demand for our TMV technology in next generation high density PoP applications. Amkor has qualified the technology for high volume production and we are now completing customer ramp readiness and SMT validations.
In addition to our customer specific work, we have presented 3 TMV joint project studies on SMT stacking and board level reliability at industry conferences over the past 2 years. We have demonstrated robust SMT stacking with standard dipping flux, paste and BGA underfill materials. We plan on offering a 14x14mm daisy chain TMV PoP test vehicle through Practical Components sometime in Q2 2010. Prior to that we will entertain joint projects with this test vehicle under non-disclosure agreements to validate compatibility with new SMT materials.”
Click here to learn more about the Amkor’s TMV and PoP package family.
TMV is a trade mark of Amkor Technology, Inc.
Dr. Andy Mackie recently put together a model to determine the probability that a component can be successfully dipped in solder paste or flux. Here is a little more from him on this subject:
"A customer in Asia was asking why one of our no-clean package-on-package fluxes, the ultralow residue NC510, was not allowing the PoP device to be picked up from the dipping tray. It turned out that the customer was allowing the flux to coat the whole of the bottom of the component, not just the solder bumps, so the vacuum nozzle had insufficient force to extract the PoP package from the flux . I got thinking about how I would model this from a physical viewpoint.
If the downward force (weight of component plus tack of flux) is greater than the upward force (air pressure on the bottom of the component), then the component could not be extracted from the flux. The figure shows the different variables. Expressing this mathematically, this comes out, in SI units, as:
Downward force = m.g + n.Ft.pi.(d/2)^2
where Ft is the tack force in units of mass per unit area, taken from the maximum tack force determined by the Tack Test Method from J-STD-005, ANSI/IPC TM 650:2.4.44
Upward force = 101000.A.pi.(D/2)^2
where A is the measure (fraction) of atmospheric pressure and denotes how good the vacuum is (zero vacuum is 0.0 : hard vacuum is 1.0).
There are some uncertainties with this approach: How does the vacuum vary across the nozzle diameter? Does the 5mm diameter flat IPC probe equate to a much smaller sphere? and so on, but it at least puts us in the right ballpark.
Just to give you a feel for how this works, the second figure shows some data. Note that scenario iv is the only one showing problems (negative force balance). The data implies that you are only likely to see an issue with inability to pick up PoP components from a dipping PoP flux tray if either:
- Components: Heavy and have many large PoP solder bumps
- Vacuum Nozzle: Too small and the vacuum is weak/poor
- Flux: Very tacky (high tack force)
and certainly, if the customer dips the whole bottom of the component into the flux, this opens up a lot of issues, including reliability (SIR); component displacement during reflow; as well as inability to pick up the component from the tray. This is why we always recommend a flux dipping height of 40-50% of the PoP bump height, to eliminate these issues."
I have found this model not only interesting, but useful for technicians to use when asked why components are 'only dipped 50%'. As a technician, it is good to have a scientific reason to refer to - even though experience may have already proven the theory to us personally.
A lot has changed in the world of package-on-package in the last few years. The most obvious change that I have seen is the development of specialized pastes for component dipping. If you haven’t tried one of these pastes, here are 8 reasons why you should:
8) More consistent transfer over time
7) Head-in-pillow elimination
6) Better wetting to a range of alloys
5) Optimized metal loading
4) Specially designed powder distribution
3) Halogen-free flux formulations
1) Higher possible yields
If you're new to PV module assembly (tabbing in particular), you're probably trying to get a feel for all the needed materials. This part is a little like getting all the materials you need to cook dinner. You need to get EVERYTHING that is required – so you don't have to go back to the store for ¼ cup of milk (or in this case, a pint of flux). In previous posts we've talked about tabbing ribbon [1, 2, 3],
Diagram of Solder-Dipping Test Apparatus
Solder-wetting Curve
Interesting question from a Power Semiconductor customer this week about how to test solder paste for use in Power die-attach applications. The customer wanted to use a wetting balance to measure the solderability of the flux. A schematic of a solder wetting balance is shown on the right: a coupon of known size is dipped in flux and then placed in a holder that grips the coupon at its top. The holder can measure the y-axis force on the coupon. Initially, this force is due only to the weight of the coupon, plus the weight of the flux. The force changes over time as the coupon is immersed in the liquid solder, initial buoyancy being followed by wetting of the solder to the specimen coupon (see second diagram: solder wetting curve).
Are you looking for information on semiconductor packaging materials? Send your request to jhisert@indium.com. It’s all fair game – released, experimental, or competitor materials. Flux characteristics, paste properties, application methods…
Inquire about any of the following topics:
A set of wires prepared for connection
Properly joined thermocouple wires
Thermocouples do a lot for us. We use them for profiling reflow ovens, checking material temperatures, and a host of other temperature related measurements. They see a lot of abuse and they are bound to break with enough rough use. So how do we fix broken thermocouple wires? I have a method that works very well. It may not be the cheapest way to fix the wires but it has the following advantages:
This method can also be used to convert leaded thermocouple wires to pb-free.
Here’s how:
1) Clip the ends of the thermocouple wires so they are even
2) Strip the sheathing back as shown in the picture (1/4inch)
3) Use a razor blade or emery paper to scrape the oxide layer off the wires, then twist the ends together
4) Put a very thin (~.001”) layer of NC 506 flux on the surface of a ceramic coupon and the exposed thermocouple wires
5) Place an 80Au/20Sn preform or a sphere(s) of the correct volume on the flux layer
6) Place the coupon onto a hotplate set to 400°C
7) Bring the wires over to the Au/Sn (which should now be molten)
8) Dip the wires into the solder
9) The solder should wick onto the wires, when it does – remove the wires.
You can leave the no-clean flux residue on the wires, or wipe it off using a solvent and rag. You now have a high-temp pb-free thermocouple.
If you'd like to discuss this with me, click here or just give me a call @ (315) 853-4900 x-7592.
From a mechanical perspective, larger solder joints are generally preferred when assembling package-on-package (PoP) components. Just as a large set of gears can handle more power, fortified interconnects add a measure of reliability to BGAs and CSPs. That is why in many cases, dipping paste is used instead of PoP flux for package-on-package stacking and BGA rework - to increase solder joint volume. The added volume of solder helps keep the solder spheres in contact with interconnect pads throughout the reflow cycle, combating the effects of warpage. This will help you increase the solder reliability and the final yield of your PoP assemblies.
Here are some links to learn more about the PoP solder paste process:
Control Your Materials, or They Will Control You (part 1)
Control Your Materials, or They Will Control You (part 2)
Package-on-Package Paste Leveling (1/5)
Package-on-Package Component Dipping (2/5)
Package-on-Package Transport (4/5)
Package-on-Package Reflow (5/5)
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.
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.
Package-on-Package (PoP) use in the SMT process is slowly creeping from a niche application into mainstream use. From experience, I know that some customers are using off-the-shelf Wireless Local Area Network (WLAN) and Bluetooth Personal Area Network (PAN) integrated into their current PCB designs. Systems that already exist suddenly become user friendly with the addition of easy Bluetooth, such as portable navigation systems or even my new motorcycle helmet. So, with that, I'd like to try something different, and post an interview with our Semiconductor Application Engineer, Jim Hisert, and resident Package-on-Package expert. Jim, what is the biggest issue facing the widespread use of PoP's in the world today? Well, Mario, it is a different process. I would say component dipping is "easier" than standard SMT into solder paste, because customers are just so accustomed to paste printing. There is also a limited material set and knowledge base for this process, so I find many people feel they are pioneering and become uncomfortable. We try to provide as much knowledge as we can up-front to shorten the learning curve. Lets talk about the component dipping process. Can you diagram the process of PoP use in the SMT world? There are actually two different process sequences that are mainstream. Process "A" -Stencil print solder paste on the entire board (this will be used for all standard SMT components and the bottom component in the PoP stack). -Place standard SMT and bottom PoP components. -Dip second level PoP component in PoP solder paste or flux. -Place second PoP component on top of bottom PoP component. -Repeat if necessary for +2 level component stacks. -Reflow. Process "B" -Stencil print solder paste on the entire board. -Place a pre-assembled PoP stack along with standard SMT components (They can be purchased pre-assembled by the manufacturer in some cases, although this limits the modular application flexibility of separate PoP components). -Reflow. Based on your experience, what can we do to help existing and theoretical processes? There are so many things to do! We need to evaluate existing materials, new materials, and competitor materials. We should keep track of new process developments and possible issues to share with our customers to make their life easier. It is important to help customers select materials, and then be there on the line to help set up the process and get the equipment settings dialed in. I feel that many customers take too much on by themselves; why "re-create the wheel"? Thank you to Jim Hisert, Semiconductor Application Engineer for Indium Corporation. More information may be found at Jim's Semiconductor Blog or Online Help: Indium Knowledge Base.
Head-in-pillow defects related to solder paste or flux performance are classified as Materials Issues. These include poor transfer efficiency on standard apertures, insufficient wetting (fluxing) capacity, low oxidation barrier and low activity. The key to over coming head-in-pillow defects is to get each component sphere to contact, and stay in contact, with the soldering material, mainly the solder paste. If the solder paste itself has poor or inconsistent transfer efficiency, then how do we know that there is even going to be contact between the sphere and the paste? Low area ratios can account for lot of the transfer issues, especially if the stencils are not electro-polished or Electro-formed (e-fab), but with that said, you must match the material set to the process and stencil design. The solder paste can cover a lot of overlap. The second half of the solder paste equation is the fluxing action. There are three parts to this; activation, oxidation barrier and stencil / tack life. High activation is an obvious choice because this is the working part of the flux, which removes the oxides from the solder and the spheres. Oxidation barriers, such as a higher rosin content of the paste's flux, are useful because it will protect the alloy from forming new oxide, which means there's more activation for the component's oxide. Also, it usually adds tack, which is a huge benefit. Because if the paste stays tacky, and the package does warp, the paste will stretch to provide a continuum, so the solder and component will be a single alloy mass upon reflow. There are artificial ways to add an oxidation barrier and additional activation, such as nitrogen reflow or a flux / paste dipping process. Nitrogen reflow PREVENTS the formation of additional oxides during the reflow process, but does not REMOVE oxides and hydroxides already formed on the components. Flux or paste dipping are viable options because this adds activation directly on the component, rather than leaving it to chance on the board. Plus, this flux or paste can be used for rework on the back-end as well. Of course, Material solutions can overcome both the Supply and Process Issues. More information may be found at Online Help: Indium Knowledge Base.
If you are new to Package-on-Package solder paste, expect the material to be lower viscosity then you are used to for printing or even dispensing applications. The PoP dipping process demands maximum solder transfer, which is provided by a different rheology than stencil printing paste. Pictured here is a new Package-on-Package solder paste. You can actually pour it out of the jar into a flux reservoir!
De-gassing a flux changes its properties so much that we can’t sell it as the same flux anymore! Beyond eliminating air bubbles, the main things that happen when you airlessly package a flux are: 1) Viscosity drops, 2) Tackiness usually drops a little – but remains similar (which influences visc./tack ratio), and 3) The flux becomes more uniform with a smoother consistency. 4) Workable life tends to increase.
The real advantages to this are the visc./tack ratio and smooth consistency. A flux with low viscosity and relatively high tack allows a chip to transfer maximum flux on each sphere after dipping. It also holds the chip in place better – with lower initial placement force. The smooth consistency resists separation and promotes uniform chemical balance from bump to bump across the chip. The increase working life allows a material to work over a longer time without a noticeable change in transfer efficiency.
Airless packaging is a huge advancement in flux manufacture, and it allows dipping applications to be possible where only spray applications could work before.
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