Building Long-term Reliability into Printed Circuit Boards

By Mike Carano, OMG Electrochemicals Inc.

The threat to the reliability of printed circuit boards (PCBs) comes primarily from one source — the temperatures required during the soldering processes for the assembly of the components onto the PCB. These reliability issues can be mitigated if fabricators understand the ramifications of the material sets chosen for the build as well as the chemical processes used in PCB fabrication. Some issues include PTH drilling parameters, multilayer board lamination, circuit density and PTH and blind via aspect ratios. Board lay-up and copper weights can influence the incidence of barrel cracking, innerlayer separation and Z-axis expansion. Some remedies include carefully matching the CTE of the material with the CTE of copper (as close as possible). Another option is to use higher performance materials with lower CTE and higher Tg. Of course, the additional cost must be considered when using these higher performance resin systems.  

In addition, which plating parameters have the greatest influence on long-term PCB reliability? Chemistry is certainly a factor but mechanical parameters including rectification, cell design and addition agent controls. Should one convert all of the plating cells to periodic pulse reverse plating? What will that move buy you? Are there other options to improve the plating process?  It is well known that improved overall plating distribution has many positive effects including improving the overall reliability of the plated-through holes and blind vias. Exceptional throwing power of the acid copper plating process is critical in providing increased thermal shock and thermal cycling resistance. One can improve throwing power with improved plating cell designs including anode configurations, proper selection of the rectifiers and the overall electrical connections. If set up properly, use of pulse plating with certain complex board designs will improve plating distribution and throwing power.

Laminate materials present special issues related to innerlayer treatment, lamination and electroplating. Some laminate materials are more brittle than others. This can reduce the bond strength in the multilayer boards. How should the fabricator approach the use of these more brittle materials in order to insure optimum reliability? Is conventional oxide more or less reliable than oxide alternative processes? To insure higher bond strengths with these more brittle materials, fabricators should play close attention to issues such as moisture absorption (both the innerlayers and pre-preg), instituting prelamination bake cycles of the innerlayer materials, and using longer lamination cycle times to insure proper resin flow and encapsulation. In addition, these higher performance materials require one to look at the operation of the innerlayer treatment process (conventional oxide versus oxide alternative). Here, the oxide or alternative oxide must be uniform and as thin as possible to give an optimum thermally heat resistant multilayer bond.

Finally, laminate materials contain fillers and other materials that can lead to poor plating adhesion on these materials. The fabricator must understand this and be prepared to use different methods to desmear the resin prior to plating. Drilling parameters must also be adjusted.

Mike will address these issues and more in his workshop on Building Long-Term Reliability in PCBs: Materials, Plating Processes and Innerlayer Treatments for Lead-Free Assembly Requirements, taking place at IPC APEX EXPO on Sunday, February 26.

One Comment

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