Solder Bridging on BGAs

What causes solder bridging on BGAs? Could it be moisture sensitivity, excessive solder paste, mismatched reflow profile or something else?

G.A.

Expert Panel Responses

Your answer is yes, all of the above. Each variable has to be taken into consideration when they are selected to manufacture and reflow BGA components. The types of flux in the solder paste needs to be evaluated for its ability and capability to absorb moisture and this will define part of the thermal profile ramp up, as any absorption has to be dried out for good reflow. Secondly excess solder paste and the solderability of the paste is important as we need the solder paste to coalesce into one solder ball at the joint of the BGA component. If the solder paste is not solderable it will short to the adjacent pads. The profile is important as it impacts how the board and the BGA component will warp and twist during the reflow cycle. Plastic BGA components will warp and twist and when they push down on the board they compress the solder paste and shorts are created. This is typically seen at the corners of the devices. If you have popcorning, or moisture in the component, the belly of the component will blow out and when this occurs the middle of the component will push against the solder paste and you could end up with shorts in the center of the component. Design of Experiments should be conducted on the products to determine the best operating parameters for the reflow profile on BGA components.

Leo Lambert
Vice President, Technical Director
EPTAC Corporation

At EPTAC Corporation, Mr. Lambert oversees content of course offerings, IPC Certification programs and provides customers with expert consultation in electronics manufacturing, including RoHS/WEEE and lead free issues. Leo is also the IPC General Chairman for the Assembly/Joining Process Committee.

Solder Bridging in BGA packages could be due to any of the following reasons PCB Related

Pad Size and Spacing
No solder mask separating pads
Solder Mask misalignment around the pads
Solder mask thickness

Component Related

Moisture sensitivity and component substrate warpage - Improve storage and handling
Solder ball coplanarity error

Solder Paste Printing Related

1:1 Aperture size (100% of pad size) - Need reduction
Contaminated stencil underside leaving smudged paste print
Excess paste due to stencil thickness and solder mask thickness issues
Solder paste slumping (due to excess pressure and T&H conditions)
Excess flux and flux separation
Improper board support during print process resulting in excess print
Improper gasket formation when using electroform stencils
Print misalignment
Paste smear due to mishandling

Placement Related

Excess placement pressure
Improper board support during placement
Placement misalignment smudging paste onto the mask
Solder ball coplanarity error

Reflow Related

Component and PCB warpage - excess heating
Excess flux activity
Component weight pushing down on the molten solder ball (if using a heavy component)

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.

When you inspect with an X-Ray you can diagnose the different causes of the bridges that you observe. For example; Moisture Sensitivity - You will see large bridges towards the center of the package. Excessive Solder Paste - Random bridges throughout the array. Mismatched Reflow Profile - You might see areas that have not reflowed, or if too fast and too hot areas of solder paste separation. Corner Bridges - Too much heat from the top causing the weaker corners to flex downward bridging the corners.

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.

From a stencil printing perspective bridging can be the result of a poor print (poor stencil) or a good print but too much solder volume. Inspection after printing will determine which is the cause. The other consideration is the placement pressure. Excessive placement pressure can cause paste to bridge from one ball to the next.

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.

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