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.

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.

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)

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.