Simple Yield Improvement. (Part 2)
In the previous post we discussed what happens to the printed circuit board structure when it is heated. We discussed how different materials expand at different rates and the effect the expansion rates have on the warp and twist of a Printed Circuit Board (PCB). We also discussed how moisture absorption can increase the degree of warp and twist and the adverse effect it has on assembly yields and potential field failures. We also discussed how dry baking removes moisture from the PCB and how it minimizes if not eliminates potential problems.
In this post we shall discuss the stabilizing effect that dry baking has on pad cratering.
With the introduction of Lead-Free assembly temperatures, solders and copper clad laminates we have discovered several new assembly non-conformances. One of these is known as Pad Cratering. Pad Cratering occurs at the corner of a large Ball Grid Array (BGA) device. The solder pad that the BGA solder ball is soldered to shears away from the surface of the PCB. The location where the pad was formerly attached to takes on the appearance of a meteorite strike and looks like a meteor crater. Pad Cratering is the result of several factors.
- The properties of Lead-Free solders.
- The inherent material weakness of Lead-Free compliant laminates.
- The size of the BGA.
- The amount of warp and twist.
Tin/Lead solders are softer and more forgiving compared to their Lead-Free solder counterparts. Phenolic cured Lead-Free compliant laminates are mechanically weaker and more brittle than their Dicy cured copper clad laminate counterparts. These are important factors to note in order to understand the failure mechanism for Pad Cratering.
When the BGA is placed onto the PCB both are at ambient (Room) temperature. As the PCB is processed through the assembly line the PCB and BGA are gradually heated. As the PCB and BGA are heated the materials the PCB and BGA are made of expand. Some warp and twist may occur. If moisture is present then the amount of warp and twist increases. As the PCB and BGA rise above the melting point of the Lead-Free solder the solder paste flows and the solder ball collapses. The solder joints are formed and the PCB and BGA are gradually cooled.
As the materials cool they contract towards their original size. As materials are heated they expand at different rates. Likewise, when they are cooled down the materials contract at different rates. When the solder lowers below it’s melting point the solder solidifies again. However, after the solder solidifies the PCB and BGA are still cooling down and contracting at different rates. As the temperature of the PCB and BGA cools from the point the solder solidifies towards ambient temperature the stress starts to build between the pad and BGA solder structure.
Another factor in the cool down process is the Tg. Above the Tg the laminate is soft and pliable. When the laminate cools down to a point below the Tg the laminate becomes hard and rigid. If the PCB has warp and twist as it cools below the Tg its shape is locked in. This is important to remember when installing the PCB in a higher level assembly as discussed below.
Under the BGA device near the center of the array the strain build up is very low. Near the center of the device the difference between the rates of expansion and contraction are at their lowest. The further away from the center of the BGA device the greater the stress becomes. This is because the difference between the rates of contraction increases towards the edge of the BGA away from the device center. The difference is greatest at the corners of the BGA since this is the furthest point from the device center. It is no surprise that Pad Cratering occurs at the corners of BGA devices.
As industry professionals review the instances of Pad Cratering, one observation has been made. The thicker the PCB the lower the potential for Pad Cratering becomes. The obvious recommendation is to make the PCB thicker. Thicker boards resist warp and twist more readily. However, thicker boards are more expensive. This increase in thickness may also not be possible since some PCB designs must stay within a specific thickness range such as a PCB that plugs into a card edge connector. The obvious solution is to minimize warp and twist.
Why is this important?
Assembled boards with warp and twist are mechanically flattened when installed in upper level assemblies. The flattening process at the upper level assembly adds more stress to the solder joints formed between the installed devices and the PCB. Prior to installation the devices may be flat while the PCB has a warp and twist to it. When the PCB is flattened when installed, the devices have to bend since the solder joints pull on the device when the PCB is flattened. The device wants to return to its point of equilibrium and return to a flat state. This force is placed upon the solder joint. The larger the device the greater the amount of force applied on the solder joints. On a BGA device the bending force applied increases towards the device edges and is greatest at the device corners.
The BGA device is essentially a spring stretched out waiting to snap back into its original shape. Is it any wonder that when you drop a smart phone or tablet from waist height that is stops working? The impact with the ground applies enough force for the spring to snap back into its original shape. The point of failure shall be at the weakest point of the soldered structure. This point of failure is typically at the laminate under the copper pad the BGA is soldered to. Keep in mind that the Phenolic cured laminates are mechanically weaker. Thus is Pad Cratering produced.
Moisture absorption into the PCB increase the amount of warp and twist. Dry baking acts as a form of stress relief because it minimizes the warp and twist of a PCB. Less warp and twist means less bend force applied to the solder joints. Less bend force results in a lower potential of Pad Cratering.