Vendor Spotlight: A solution to pad cratering.

Lead-free assembly techniques and practices have pushed the envelope of printed circuit board technology for years. The industry has been forced to change and improve as a result of customer demands and regulatory compliance. Lead-free compliance has been a journey filled with advancement and disaster. The advancement comes in the form of new improved materials and surface finishes. The disaster came in the form of all of the manufacturing non-conformances found along the way and yet to come. Over the past six years the electronics industry has been one giant crucible. Within this crucible we have tested the limits of the process at the bare board and assembly levels.

Prior to lead-free assembly practices there was eutectic solder consisting of tin-lead (Sn63/Pb37). As the saying goes “Nothing solders like solder”. Eutectic solder wicks easily at relatively low temperatures. Solder joints have a unique property where the inter-metallic copper-tin alloy grows over time. There are decades of historical information and observation on the longevity and durability of eutectic solder joints and assembled materials. In a competitive industry where change introduces the potential of non-conformances, eutectic solders are a proven and safe technology.

Enter lead-free assembly. We have no long term data on the effects of lead-free assembly techniques, components, materials and solders. Whereas we have decades worth of eutectic soldering data, we have virtually nothing on lead-free soldering. There are only a few years of data in the field. Most of us don’t realize it but we are already using lead-free devices. I recently went to Best Buy to buy a cordless phone. I try to avoid lead-free devices whenever possible. My greatest fear has been realized. All the phones had a “Lead-free” compliant emblem on the side of the box. I came home with a cordless set of four phones. My family likes to drop the phones so we have a running bet on which prone shall fail first.

Why am I concerned with which of my phones shall fail first? Cell phones and hand held devices have to meet an industry drop test. We know what happens with eutectic solder joints and traditional materials used within the eutectic soldering realm. We have all been using devices made with eutectic solders for years. We know how durable they are. Lead-free has no long term history in the field. Impact tests and long term simulations tell us something. However, there is no substitute for test results from the field. We are all currently test subjects in the lead-free experiment. We are currently using lead-free devices that have an unproven track record in the field. Hence my reference to the crucible.

Lead-free solders are more brittle than eutectic solders. These solders also require higher soldering temperatures for extended periods of time. Traditional grades of FR4 are not suited for these soldering applications. They degrade and breakdown at higher temperatures. Phenolic grades of FR4 are better suited to take the heat of assembly. Unfortunately these laminate materials tend to be mechanically weaker. This leads to the potential of a mechanical non-conformance. No device has proven more susceptible to this type of potential field failure than the ball grid array (BGA). BGAs are very difficult to assemble. A large flat device with hundreds of tiny solder balls soldered to the surface of a printed circuit board. Assembly profiles can be very challenging for these devices.

Materials expand when exposed to heat and contract when cooled. It is important to understand that materials do not expand and contract at the same rate. This difference in the rates of expansion and contraction introduces added stress to the devices. Stress is introduced as the materials are heated and expand and again as the devices cool down and contract. When a BGA cools down the balls on the edges stabilize and solidify first. The balls under the center of the device cool down at a slower rate and shall stabilize and solidify last. The printed circuit board may also expand and contract at a different rate. Another variable is that the design of the printed circuit board may result in a slight twist as its heated and a flattening as it is cooled. An unbalanced construction or a split power plane may be a source for this.

Materials want to attain a point of stress free equilibrium. Once the solder joints solidify they are still under stress introduced by the different rates of contraction from the point of solidification. When a lead-free device is dropped something may give. One of the solder balls under stress may develop a stress fracture since the lead-free solder is more brittle. A stress fracture could form in the laminate under the pad the BGA is soldered to since the laminate material is mechanically weaker. The force of the drop is the catalyst that may provide enough force that pushes the stress over the edge and starts a fracture.

Devices in use generate heat. As they heat the materials expand. As the devices use less power or are turned off they cool down. The materials contract. As the fracture is subjected to stress introduced from the constant expansion and contraction of use the fracture grows into a crack. The crack continues to grow until the unit fails. The device stops working and is technically a field failure. Welcome to life in the crucible.

Lead-free assembly has introduced many challenges and non-conformances. One of the more troubling non-conformances is “Pad Cratering”. Pad cratering is when the copper pad completely separates from the laminate. The separation may occur outright at assembly or as the result of the field failure scenario detailed above. Pad cratering is the result of the technology exceeding the capabilities of the materials.

Pad cratering.

Example of pad cratering provided by Integral Technology.

To date there has been no solution to the problem of pad cratering, until now. Integral Technology has developed a solution that has the potential to minimize if not eliminate pad cratering. Integral’s solution is a material called Zeta®. Zeta® is a high performance polymer film that is capable of withstanding high temperatures. It has no woven fiber-glass thus making it CAF resistant. It can be used in combination with current technology as an additional material layer between the outer layer foil and outer most dielectric. Zeta® has a high mechanical strength and flexibility compared to other lead-free compliant laminate materials.

Zeta Cap

Zeta Cap provided by Integral Technology

It is the higher mechanical strength and flexibility that make this a solution for pad cratering. The higher mechanical strength (6 to 7 pound peel strength) helps to prevent the BGA pad from peeling away. The improved material flexibility acts as a stress relief generated by the forces previously discussed. There is less stress on both the laminate and lead-free solder joint as the result of this flexibility. When I need to replace my cordless phone maybe the next one I buy shall have Zeta® in it. There are other potential applications for Zeta® in the world of HDI as well. The two images above are referenced from Integral Technology’s slide presentation on Zeta®. The following link is a copy of the full presentation and its potential applications.

zeta-2-15-11

This material has the potential to be a real game changer. For more detailed information on Zeta® and other product solutions from Integral Technology contact…

Jim Griffin
OEM Marketing Manager
Integral Technology
office: 603 629 4420
cell: 603 738 5821
jgriffin@Integral-HDI.com

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