Over the last few years I have been asked to examine examples of Ball Grid Array (BGA) defects which are referred to as dropped ball. This type of defect can be found at different stages in the manufacturing process depending on its route cause. It can be difficult to detect even with under package optical or x-ray inspection as the separation gap can be extremely small.
First, it’s important to confirm what “Dropped Ball” really means in practice.
“Dropped Ball – The loss of one or more balls from the base of an area array component and found during inspection of the component or after first or second side reflow”, this seems appropriate as a definition, but many people may have a different description.
If you review other literature dropped ball has also been used to define:
Balls separating from the package interface during drop testing, vibration, flexure or temperature cycling. Obviously, this is the point of doing environmental testing to find the weakness of the interconnection or prove the reliability of the joints.
Balls separating from the package in the early years of BGA use during handling and placement. This is less common today due to the control of area array manufacturing process and the quality control procedures in component assembly line. Automatic optical inspection is used along with sample shear testing of the ball to substrate interface after reflowing.
Each type of ball drop can generally be illustrated by the way the ball separates from the surface of the package termination pads. In the case of drop testing or shear testing the failure site will have a hard-fractured surface on the pad and the ball, just like a jigsaw puzzle
If the balls drop during reflow or rework the surface of the pad will probably still be perfectly wetted with solder. This would happen if in a liquid state the solder surfaces are stretched and separated. The appearance of the ball or solder will also look different if separated in air or nitrogen environments. Ball drop could also occur and has been seen on BGAs running over a wave soldering system with top side preheat, however the ball may not be round it may be elongated when examined.
During normal reflow low temperature balls will become liquid and be in a liquid state for a period during reflow. As the solder ball is already attached to the package it will mean a change in the thickness of the intermetalic layer between the alloy of the ball and the nickel. The gold will have gone in to solution during the original ball attachment process. If the solder paste and the ball have the same alloy mixture there will be little to indicate any change. If a different ball alloy is used like tin/silver or tin/silver/copper, there would be a difference in the final joint alloy due to the paste that is used. If tin/lead paste is being used this will probably be the most noticeable.
When soldering a BGA to a nickel/gold board you will see a thinner intermetalic layer at both joint interfaces as nickel is not very soluble in tin. If the surface of the board is copper, silver, tin or solder levelled you will see evidence of more copper at the interface. Simply copper is more soluble in tin, even more so with higher process temperatures and higher tin content solders.
In the case of double sided reflow the balls could reflow again, but this would depend on the set-up of the oven profile and the ball metallurgy. Normally if balls reflow on the second side they only change slightly in shape unless the board or BGA package warp. If the BGA warps during first or the second reflow operation you would expect elongated or compressed joints. During wave soldering the topside of the board should not be allowed to reflow again. BGA terminations can be separated in this way but normally only on the board surface and not the package.
Area array packages that use solder mask defined pads may exhibit some specific issues during soldering. The solder mask could expand or lift from the surface of the component at elevated temperatures. This could cause the solder ball in a liquid or semi state to become detached from the pad surface. Satisfactory wetting of the package pad would still be present, and the ball may still have a rounded shape rather than a rough or brittle surface. The shape of the ball and the surface of the pad would also be dependent on the use of nitrogen in the reflow process.
It’s important to remember that is you are going to conduct failure analysis on problems of this type it’s important that all the process information is provided with the sample. Its also crucial that the board assembly has not be subjected to any other examination prior to review as this make the true cause of failure difficult to determine. Details on Failure Analysis training workshops, online consultancy and support are available from our website