Plating anomalies and defects, Part 1: plating defects can have multiple origins, so don't jump to conclusions.
Blisters (Hole-Wall Pullaway)
The common complaint one hears is, "The copper plating is peeling." Ok, but where is it peeling from, off of the copper surface or from within the via? What about the interconnect? Is the peeling layer the electroless copper deposit or the electrolytic copper? These are the questions one must answer in order to properly trouble shoot the defect. FIGURE 1 shows a real-life example of a blistered or peeling deposit. The copper deposit has actually flaked off or blistered from the hole. In some cases, the deposit does not completely flake off the surface, but instead, pulls away from the hole wall. This condition is referred to as hole-wall pullaway (HWPA).
Now the question is where is the origin of the blister? In this case, the good news is that we are only looking at the electroless copper. There has yet to be an electrolytic copper deposit applied to the circuit board, but that is all one can tell at this point.
A more typical example of HWPA is depicted in FIGURE 2. The deposit pulled away from the hole-wall but did not fracture or flake off. Nonetheless, this is a defect and must be remedied. The root cause of HWPA is very similar to those that lead to flaking and blistering.
So, one is dealing with the origin of the blister or peeling deposit prior to electrolytic copper plating. TABLE 1 lists the most common causes for this type of defect. Further information about the hole wall condition can be determined by a careful review of the microsection or through SEM analysis.
Keep in mind that there is no substitute for process control. All of the operators should have a basic understanding of why certain processes must be controlled within the stated limits. This will go a long way in preventing costly rejects.
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MICHAEL CARANO is global manager for strategic business development at OM Group, Inc. and can be reached at firstname.lastname@example.org.
TABLE 1. Common causes of NWPA. Cause Look at Excessive catalyzation Dwell times, temperatures Smooth topography on resin * besmear process not able to produce a topography * excessive solvent penetration * Insufficient rinsing after cleaner/conditioner step * Inadequate etching of the resin-poor topography Electroless copper deposition * Check rate coupons-is dep rate too rate higher than normal? high * Check operating temperature of electroless copper solution * Check concentrations of the key additives-copper, sodium hydroxide, HCHO, stabilizer concentration Cause Remedy Excessive catalyzation Reduce time and temperatures in catalyst Smooth topography on resin * Review resin type-higher Tg resins are more difficult to desmear-excessive lamination temperatures and cure times over cross link material. * Make sure cleaner conditioner is free-rinsing--consider reducing concentration and dwell times * Measure weight loss of the test coupons after Besmear Electroless copper deposition * Check specific gravity of rate too electroless copper solution high --solution bail-out to bring specific gravity in range * Reduce operating temperature * Analyze and adjust concentrations to recommended levels
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|Title Annotation:||POSITIVE PLANNING|
|Publication:||Printed Circuit Design & Fab|
|Date:||Jun 1, 2009|
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