Issues with BGA Components Near PCB Edges

What issues are we likely to see when we place BGA components very close to PCB edges? What impact might it have on reliability? Will equipment (screening, placement, reflow, etc.) require modification?

T. B.

Expert Panel Responses

Thermal Profiling is the key to determine if you are going to have a reflow problem with a BGA, or any other component, near the board edge.

A careful thermal profile of the board and BGA near the edge as compared to the middle of the board will need to be performed to characterize the nature of the oven near its rails. Many reflow machines run several degrees cooler near the rails, and as you know, this can make the difference between a good and bad BGA reflow process.

Place T/Cs at all four corners of the BGA and one or two others at a BGA closer to the middle of the board and compare the resulting profile temperatures and times above liquidus. If you do not have the exact assembly for this characterization profile, then use most any scrap assembly with a similar component density, even cutting it to size in order to position a BGA near the edge of the board.

This will let you know if the reflow process is going to give you problems. There are several ways to solve this problem in roughly descending cost order:

Get or use a different oven.
Don't design the BGA near the edge of the board.
Build a "reflow fixture" or pallet to center the board in the reflow oven.
Leave the PCB in its process pallet with the "thief area"to space the real edge of the board away from the oven rail.
Turn the board 90 degrees so the BGA is on the leading or training edge, and not against the rail of the oven.

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.

At least on the reflow front, you can use profiling as a tool to ensure you are reaching the correct peak temperatures for larger mass BGAs, while not frying more sensitive components on your board. Also remember across the belt uniformity can range anywhere from 2 to 5C+. Often the edges of your PCB is the coolest in the oven, therefore requiring even more heat for your BGAs to reflow correctly. This makes even more critical that you use profiling as a method to "balance" your board. For example if your BGAs are normally set to a max peak temp of 245C you might need to bump this up slightly, but you better keep a close eye on components like electrolytic capacitors. With some software like KIC2000, you can redefine these sensitive components, for example, lowering their max peak to 235C. I just posted today some great pictures to show you what is being used to measure uniformity across the belt from one oven manufacturer as well as examples of balancing your board with profiling.

Brian O'Leary
Global Account Manager
Indium Corporation

Mr. O'Leary is the Global Account Manager for Indium. He has and extensive global network of contacts in the electronics industry with expertise in SMT equipment and processes.

In regards to the stencil printer, you will have to make sure that depending on how close to the edge of the board you are talking about that any PCB clamping system that is present on the top side of the board during printing is designed to not affect stencil contact at the print point or you will need to ensure that your printer has the capability to pull the board flat and then retract any top side clamping system before printing.

Steve Hall
President
EKRA America

Mr. Hall has spent the last 20 years in the electronics manufacturing industry. He started at Motorola specializing in the development of screen printing and reflow soldering processes. He has became known as an expert in printing technology.

If your printer has board edge clamps you may need to put a relief pocket on bottom side of the stencil to assure good stencil to board gasketing in the area close to the edge of the board.

Bill Coleman
Vice President Technology
Photo Stencil

For over 18 years, Dr. Coleman has been the vice president of technology for Photo Stencil, working closely with customers to understand their printing requirements. His efforts have resulted in several new stencil products.

From the PCB design side, with BGAs close to the edges, you may have problems routing the traces away from the component. From the reflow side, your higher mass BGA may require additional heat for reflow, as a result you may overheat the edges of your board during soldering. From the rework side, your BGA on the edge of the board may cause edge warping during rework of the BGA.

Edward Zamborsky
Regional Sales Manager
OK International Inc.

Ed Zamborsky is a Regional Sales & Technical Support Manager for Thermaltronics, located in New York. His position requires frequent customer visits throughout North America and the Caribbean and his position encompasses not only sales but the role of trainer and master applications engineer for all of Thermaltronics products. His expertise includes such specialties as hand soldering, convection and conduction reflow techniques, array rework, fluid dispensing equipment, and fume extraction. Ed has authored many articles and has presented many papers on topics such as; Low Volume SMT Assembly, Solder Fume Extraction, SMT Rework, BGA Rework, Lead-Free Hand Soldering, High Thermal Demand Hand Soldering, Lead Free Visual Inspection and Lead Free Array Rework.

When the PCB is post-assembly separated frompallet/panel form to individual form, the various methods of"depanelizing" will load different levels of stress along the PCBedges (e.g., V-Score vs. tab-rout, both individually and in combination). As SMD distance to board edge decreases, component solder jointsusceptibility to the mechanical stresses of depanelization increases. Can'tpredict joint force loading through modeling? Have your PCB house provide a"print & etch" sample for practical evaluation.

Robert "Bob" Lazzara
President
Circuit Connect, Inc.

Bob has been in PCB design and fabrication since 1976. He has held elected positions with the SMTA, is a member of the MSD Council, has served as a committee member for various IPC standards and is a Certified IPC Trainer.

There are a number of other factors to consider whenplacing BGAs closer to the edge of the PWB other than those already mentioned.Here are some other considerations:

The edges of the board are nearly always 5-10 degreesHOTTER (not cooler) than the center area if any heatsinking due to fixtures(pallets), rails, etc, are removed from the equation. On the other hand, theedges can be a good 5-30 degrees C. COOLER than the center of the CCA if it isresting on a conveyor or pallet along its entire edge on two or more sides.Reference IPC-7095, it has a lot of good information on this subject as well asmany other BGA design considerations and guidelines.
To ensure that the edges are not affected byheatsinking along the entire sides of the CCA, such as during reflow from thereflow pallet or edge conveyor, or from the board holder during rework,"fingers" should be designed into the pallet to prevent the heatsinking. Thesefingers extend out from the pallet to hold the board by utilizing the toolingholes in the board. While they prevent heatsinking that causes cooler edges,their use WILL give you a slightly hotter edge (typically 5-10 deg. C hotter).This might be easier to deal with than having the edges as much as 30 degreescooler, however.
Another reason not to put the BGA package closer tothe edge is because after you pin out to all of the latching caps, etc, andtheir vias, you have too many other items that can be damaged due to flexure ofthe PWB during depaneling; the fractured BGA solder joints are not the onlyconcern.
Local BGA fiducials may not be able to be accessed bythe pick and place camera if they are too close to the edge.
BGA rework nozzles need clearance also.
There may not be enough room for the BGA test pointsthat you will want to add later.
If you think warpage in BGAs is bad, wait till yousee what happens when you have at least a 5-10 deg. C delta T (temperaturedifferential) between the side of the BGA closest to the edge and the sidecloser to the center of the CCA.

Richard D. Stadem
Advanced Engineer/Scientist
General Dynamics

Richard D. Stadem is an advanced engineer/scientist for General Dynamics and is also a consulting engineer for other companies. He has 38 years of engineering experience having worked for Honeywell, ADC, Pemstar (now Benchmark), Analog Technologies, and General Dynamics.

I would not stressthat much about this. However- few things to consider:

Heatneeded to reflow the parts - a KIC or MOLE profiler will give you what you need
Boardhandling (printing, pick and place, reflow) - design to be considered (rails,etc.)
Depanelprocess - this is also important as solder fractures can occur if the solderjoint is close to the edge and a mechanical process is used for separation ofthe rails - a router is preferable against a V-score depaneler or a tab cutter.

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.

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