Tombstoning is often caused by one end of the part reaching liquidous 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 nearthe 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 liquidous sooner in time then the other end:
- The one end of the component enters the oven'szonebefore the other end due to the board geometry.
- 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 liquidous 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 liquidous 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 liquidous 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 liquidous 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.
Senior Project Engineer
Electronic Controls Design Inc
Paul been with Electronic Controls Design Inc. (ECD) in Milwaukie, Oregon for over 34 years as a Senior Project Engineer. He has seen and worked with the electronic manufacturing industry from many points of view, including: technician, designer, manufacture, and customer. His focus has been the design and application of thermal process measurement tools used to improve manufacturing processes like: mass reflow and wave soldering, bread baking, paint and powder curing, metal heat treatment and more.
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.
- Uneven Pad Design
- Microvia 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
- 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 offset or misalignment
- Insufficient pressure
- Nitrogen reflow more prone than air reflow
- Convection flow rate
- Improper heat distribution
- Rapid ramp rate
- Short soak time
Bjorn Dahle is the President of KIC. 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
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.
Dr. Brian Toleno
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.
Principal Engineer - CMA Lab
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.
Paul J. Koep
Global Product Manager
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.
Technical Sales Manager
BLT Circuit Services Ltd
Greg York has twenty two years of service in Electronics industry. York has installed over 350 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.
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 TroubleshootingFrom 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.
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)
Operations Manager - A&E, AWS
Projects Unlimited, Inc.
Georgian is Operations Manager for Projects Unlimited in Dayton, Ohio. His background is in Electronics Engineering and has experience in electronic assemblies for over 15 years.