Dipping Paste

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：

• Ning-Cheng Lee, Ph.D, Vice President of Technology 李寧成博士：

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.
 

• Ronald C. Lasky, Ph.D. PE, Senior Technologist

²       Achieving High Reliability for Lead-Free Solder Joints – Materials Consideration

• Mario Scalzo, Senior Technical Support Engineer

²       Addressing the Challenge of Head-in-Pillow Defects in Electronics Assembly.

²       Challenges for Implementing a Halogen-Free Process

• Eric Bastow, Senior Technical Support Engineer

²       Understanding SIR

• Chris Anglin, Applications Development Engineer

²       Stencil Printing Transfer Efficiency of Circular vs. Square Apertures with the Same Solder Paste

 這些文章在Indium的技術網站上面，都可以免費下載。

Cheers!

I love watching a good basketball game, and one of my favorite local teams is the Syracuse Orangemen. If you go to a Syracuse home game, notice the shot clock – it was made with Indium Corporation solder. There are a lot of places you can see our products in your everyday life. That smart phone in your pocket, the electrical components in your car, the thermal interface in the computer in front of you. That’s one of the things that makes this job rewarding, being part of so many various applications.

In addition to learning about these different applications, we also get a good reference for what assembly trends are developing, and which material technologies are becoming more popular. 

I’ve watched the halogen-free trend explode and fade, as it was adopted by some large OEMs and their contract manufacturers, but has not spread to most other companies. Another trend that is fading away from the spotlight is Pb-free die-attach solder, since the EU has not found a suitable replacement and has pushed back the exemption deadline. 

A long-existing topic that has had recent mention is solder jetting. The trend towards soldering smaller components is not new or surprising, but for smaller components (01005s and 0201s) we have seen a trend towards dispensing instead of jetting – which seems to suit those applications.

For small component printing, transfer efficiency is critical. Outside of solder paste optimization, “nano-stencil” technology is an upcoming technology that may take-off and improve paste release characteristics. Solder paste is being used in some other creative ways too, like low temperature alloy dipping paste for rework operations. Manycompanies are now using or evaluating specialized solder applications to replace components without fully reflowing the rest of the components on the board.

Integrated preforms are finding their way into more and more applications recently as well. These connected preforms are being used to reduce the need for component pallets and selective soldering operations.

All these applications are great ways that our customers are taking soldering technology to the next level, using materials and assembly methods that were not common before. I look forward to learning how you’d like to use solder in your application!

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.

For more information, Lee can be contacted at Lee.Smith@amkor.com

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. 

Brandon Judd and I have been working on a paper for this year’s SMTAI event, and I thought I would share a snip from it.  Although people generally share the beginning of a paper, I’d like to share the conclusion, in Brandon’s words:

“With the miniaturization of today’s electronic devices and the increasing complexity of their features, the need for PoP components is significantly increasing.  Although it may seem like a simple solution to just use the standard SMT paste that you have in-house for your upcoming PoP applications, these products will not be optimal for this type of process.  As our testing has shown, modern PoP solder paste materials are much better suited to the dipping process used for PoP components.  Formulation, particle size, and metal loading are all key factors in the design of a PoP specific paste.  The time spent evaluating these new products is well worth saving yourself the headaches of getting electrical opens on your boards from using standard SMT materials or even outdated dipping pastes, causing costly and time consuming rework down the road.  With the proper material and process, insufficient solder transfer and head-in-pillow defects should be a thing of the past.  “   

This is something you may have run into if you’ve ever manually dipped and placed PoP components.  Yesterday while trying to hold a conversation about the package-on-package process, I lost track of the very same process I was discussing.  I neglected to dip one of the components into solder paste before placing it on the board.  This is a picture of what happens.  The stack itself soldered well, as would be expected, but fell off when the board was lifted from the conveyor at the end of the line. 

If you’re wondering what a small error like this costs:

1 hour lost time + price of the board and components.

Luckily the other 14 PoP stacks can still be used for cross-sectioning and learning more about the PoP paste that was being evaluated.  If this was a production board I would be able to simply dip the stack and re-place it on the pads, send it through a second reflow, and test the final assembly for functionality – but you just can’t get away with that during evaluation.

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

2)       Maximized transfer volumes

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], as well as tabbing flux.  It is important that both of these products are used during tabbing.  The flux is used to remove oxides on the surface of the tabbing ribbon (the solder coating) and promote wetting to the metallization paste.  Liquid flux is generally used for this application, and it is applied by dipping the ribbon into it.  Feel free to email solar@indium.com to learn exactly how these products should be used in your application.

A side view of PoP paste Indium9.88HF after .4mm component dipping.

By propping your component up sideways under a microscope (if you don’t have one with a side-view function), you can get a pretty good view of the paste deposit.  There are different theories surrounding the deposit profile and its relationship to ‘performance’, although they are all just theories at the moment.  We have worked with pastes that have uniform and non-uniform profiles - pastes from each category have worked well.  There are so many other characteristics that impact warpage compensation, it’s probably not fair to judge a paste by its profile.  Viewing dipped components from the side does allow you to see roughly how much solder paste was applied, and if the deposit is consistent from sphere to sphere.

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).

For a change of pace, again, I have asked another Technical Support Engineer, Chris Nash, to comment about powder sizes.  Chris is the Regional Technical Support Engineer for the Midwest region, and works from Indium Corporation HQ in Clinton, NY.

What’s the difference between Package-on-Package (PoP) / BGA rework solder paste, and solder paste designed for SMT?  Solder paste for dipping applications is designed to transfer more solder based on its rheological characteristics.  In the chart shown here, a typical SMT paste is compared to 3 next generation dipping pastes.  Although the names cannot be released right now, all PoP/rework pastes transferred over 100% more paste to the solder joint area.  (Hint: you can probably break me down relatively easily if you have me on the phone.  For the whole story call me at (315) 853-4900)

This added solder volume helps ensure that more solder is available during joint formation to compensate for component warpage.  During rework, increased solder volume replaces solder that has been scavenged during the component removal process.  Either way, more solder volume relates to a more robust solder joint.

• 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:

  • Pin transfer
  • Package-on-Package (PoP)
  • Solder spheres
  • Glass transition temperatures (Tg)
  • Flip chip assembly
  • BGA rework
  • Cross-sectioning electronic components
  • Paste for component dipping
  • Solder alloys
  • Liquid fluxes
  • Wafer bumping
  • Low alpha solder
  • Spin coating
  • Redistribution layers (rdl)
  • Halogen-free
  • Flux viscosity
  • Solder paste viscosity
  • Whatever else you are interested in

 

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 Placement (3/5)

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.

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.