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February 19, 2018

Tombstoning Dilemma

We're having a problem with small chip tombstoning. The chips are lifting off one side. I know this issue has been discussed in various magazine articles and forums. I'm curious to read your input on the possible cause and how we can eliminate it.

T. P.

Expert Panel Responses

Tombstoning is often caused by one end of the part reaching liquidus temp just before the other end. The surface tension of the liquid solder at one end pulls the part upright off the paste of the other end.

It is important to confirm this by having a good thermal profile of the assembly with thermocouples attached at or near the component ends to see what the thermal nature of the assembly is in the region of the component affected.

There are several reasons why one end of a component may be reaching liquidus sooner in time then the other end:
  1. The one end of the component enters the oven's zone before the other end due to the board geometry.
  2. One end of the component is near a thermally massive component and the other not so close, making one end get hotter sooner.
Both problems can be reduced by slowing down the profile ramp rate as the parts approach liquidus and then holding the profile a little longer in the peak phase of the profile,at a slightly reduced peak value.

The goal is to allow the components to enter liquidus a little slower (reduced slope), giving more time for the entire component to heat and thus at a rate which gives both ends more time to heat the same.

With a good thermal profile of the assembly, you can make adjustments using the profiling software's prediction tools to see what oven settings can best alter the profile shape around the liquidus temp to reduce the temperature difference at each instant in time.

This may mean ramping a little faster out of the soak phase and giving the last two or three heated zones better control over the ramp rate through liquidus to peak. In other words, don't rush to peak, rather slow it down a bit and hold for a few more seconds at a little lower peak.

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Paul Austen
Senior Project Engineer
Electronic Controls Design Inc
Paul been with Electronic Controls Design Inc. (ECD) in Milwaukie, Oregon for over 39 years as a Senior Project Engineer. He has seen and worked with the electronic manufacturing industry from many points of view, including: technician, engineer, manufacture, and customer. His focus has been the design and application of measurement tools used to improve manufacturing thermal processes and well as moisture sensitive component storage solutions.

Tombstoning happens due to imbalance in the surface tension forces exerted on the terminations of small passive components. The imbalance can be due to many different factors. Here is a listing of the key ones.

PCB Related:
*Uneven Pad Design
*icrovia in one pad and not the other
  • One pad acting as a thermal sink and not the other *Variations in the surface finish
  • Variations in the levels of surface contamination
    *

    Component Related:
    * Varying degree of contamination on terminations
    * Improper plating of terminations leading to varying degrees of oxidation
    * Paste Print Related:
    WS paste more prone than NC paste
    Tin-Lead more prone than Lead-free
    Varying amounts of paste on the pads
    Improper distribution of particles and flux between the prints on the pads

    Placement Related:
    * Placement offset or misalignment
    * Insufficient pressure

    Profile Related:
    * Nitrogen reflow more prone than air reflow
    * Convection flow rate
    * Improper heat distribution
    * Rapid ramp rate
    * Short soak time

    image
    Bjorn Dahle
    President
    inspīre solutions LLC
    Bjorn Dahle is the President of inspīre solutions LLC. He has 20 years experience in the electronic manufacturing industry with various manufacturing equipment companies covering pick & place, screen printers and thermal process management.

    If you review the articles the two main, statistically significant factors are: 1. Pad Design and 2. Materials. There are manufactures who have solder paste formulations specifically designed to minimize and/or eliminate the occurrence of tombstoning (see the APEX 2003 proceedings). From a pad design standpoint, you want to make sure you are following well recognized design rules so that you have optimum coverage and placement of the component on the solder paste.

    image
    Dr. Brian Toleno
    Application Engineering
    Henkel Electronics
    Dr. Brian Toleno is the Application Engineering Team leader for Henkel Technologies. He is responsible for the technical service and application engineering for Henkel's electronics assembly materials, including solder paste, underfills, PCB protection materials, and underfills.

    Tombstoning can have a variety of causes but check for the following. Are the PEC pads on both sides identical or of different sizes or, even worse, shared with another component? This will create an imbalance resulting in uneven forces on the chip during reflow. Check that the solder paste deposits are identical in shape and volume on both sides of the chip position and the chips are being placed across the position accurately with the same amount overhanging the solder paste. Also worth checking is the orientation of the chips through reflow. You will get best results if both solder joints reflow at the same time. This depends on the design, and whether this is feasible! There are a few other factors but these areas are the favorites.

    image
    Bryan Kerr
    Principal Engineer - CMA Lab
    BAE Systems
    Bryan Kerr has 35 years experience in providing technical support to PEC assembly manufacturing. His experience ranges from analysis of materials and components to troubleshooting and optimizing, selecting reflow, cleaning, coating and other associated processes.

    The cause of this defect is inadequate contact surface area of the component metalized pads in the solder paste. This could be caused by a miss alignment of the component in the printed solder paste from poor placement, poor registration of the stencil on the pad, or less than adequate component metalization surface area in the paste. Designing the pad and stencil so that greater than 50% of both metallized leads of the chip component are making contact with the solder paste deposit is the best way to prevent tombstoning. Use of non eutectic alloys is a widely recognized solution as well, but the placement / design is a much stronger influence over the defect.

    image
    Paul J. Koep
    Global Product Manager
    Alpha
    Mr. Koep is responsible for product planning and technical marketing for the Preform Products at Alpha. He is the co-author of several patents in the areas of soldering applications focusing on reflow and alternative methods.

    Tombstoning can be designed out with Stencil/Pad layout and turning the PCB in reflow so both sides of the component receive equal heat but to be honest all these are possible but hard work. Simplest way is start to use a Low Tombstoning paste especially if used in Vapour Phase Machine. This is the simplest answer to the question. It is generally ran on a SAC305 profile but has a mix of alloys to stop Tombstoning or greatly reduce it. I know AIM Solders do one that works very well and probably others do as well so best to search out Low Tombstoning Paste on the Internet and give it a try.

    image
    Greg York
    Technical Sales Manager
    BLT Circuit Services Ltd
    Greg York has over thirty two years of service in Electronics industry. York has installed over 600 Lead Free Lines in Europe with Solder and flux systems as well as Technical Support on SMT lines and trouble shooting.

    This can be caused by many factors:
    • To rapid a rise in the preheat section of his oven.
    • Solder paste deposition inconsistencies from one side to the other.
    • Components not be centered between the solder pads.


    image
    Edward Zamborsky
    Regional Sales Manager
    OK International Inc.
    Mr. Zamborsky serves as one of OK's technology advisers to the Product Development group. Ed has authored articles and papers on topics such as; Low Volume SMT Assembly, Solder Fume Extraction, SMT Rework, BGA Rework, Lead Free Hand Soldering, Lead Free Visual Inspection and Lead Free Array Rework.

    There is much "prior art" on the subject of tombstoning and many different variables and causes. Here are a couple of articles that nicely and concisely summarize the variables and offer solutions. From SMTnet, is an EFD article, Tombstone Troubleshooting From Circuitnet, a Cobar Products article, Tips for Preventing 'Tombstoning' of Chip Components The one from the Circuitnet site is especially nice as it gives engineering analysis showing the forces involved which can give you greater understanding and insight as to the mechanics of tombstoning.

    image
    Marc Peo
    President
    Heller Industries Inc.
    Mr. Peo has been with Heller Industries for over 20 years and has been President for the past 8 years. Marc has authored several industry articles on Soldering, Flux collection, nitrogen use and Lead Free conversion.

    For sure a lot of variables in this problem:
    • Pad design
    • Paste deposition (x, y deviation and volume/release from the stencil)
    • Paste type - if you are placing 0201 and smaller components and using type 3 paste you will have this problem - try type 4 or higher
    • Combination of leaded paste and lead free component?
    • Component placement (x, y position, placement pressure)
    • Component and/or board plating quality and possible contamination (oxidation)
    • Assembly time - is the processing time extended to the level where the flux solvents are drying out (small components will have small paste depositions so less flux)
    • Reflow oven issues - profile, convection/air flow, vibration on the moving parts (belt or chain)


    image
    Georgian Simion
    Engineering and Operations Management
    Independent Consultant
    Georgian Simion is an independent consultant with 20+ years in electronics manufacturing engineering and operations.
    Contact me at georgiansimion@yahoo.com.

    Reader Comment
    Something I did not see addressed above, but might be in the published articles. We have adjusted our PCB silkscreen on our small part footprints (0603/0402)to remove the lines underneath the parts body. We found in some cases that thick soldermask and legend (UCL) acted like a teeter-totter and kept the component from contacting both paste "piles" thoroughly, supporting tombstoning. This might require careful testing to actually prove, or even cross-sectioning. Also, if the board utilizes Dry Film soldermask, this can be an even bigger concern based on it's thickness. Close optical inspection might also show this failure mode.
    Les Beller, Echostar Technologies, USA


    Reader Comment
    In my experience, in terms of Pareto, I would say:
    • 30% for pad design
    • 20% for stencil design
    • 15% for solder paste printing offset
    • 12% for placement offset
    • 7% solder mask registration on pads
    • 3% reflow profile
    • 13% everything else
    Glayson Figueiredo, Philips Medical Systems


    Reader Comment
    These are all great comments. One that hasn't been stated is the copper balancing on the component pads themselves. Copper is a great heat sink. When the copper entering the pads on each side of the component is significantly different, it affects the thermal balance.
    Cherie Litson, Litson1 Consulting


    Reader Comment
    Here is a tip that has often worked for me; deliberately placing smaller chip caps and resistors (0805 and smaller) at about 30 degrees from perfect alignment usually reduces tombstoning. For the same reason that it is much more difficult to lift a rectangular box up against it's corner rather than against either end. So if the chips are placed at a slight angle such that two diagonally opposite corners are centered on each pad, the part first has to align itself before either end can be lifted by any uneven wetting tension on one end or the other.

    This slight delay during alignment is often enough for wetting tension to approach equilibrium on both ends, reducing the tendency for either end to be pulled up. (Remember what I told you about engineering being nothing more than the calculated control of forces trying to reach equilibrium?) But for chip caps with slightly rounded corners, all bets are off!
    O. Stadheim, Viking Elektomek

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