Microetch: The Unsung Hero Print E-mail
Written by Lenora Toscano   
Friday, 29 March 2013 22:17



The right surface roughness sets the stage for subsequent plating.

My last column focused on the importance of the cleaning step in the final finish line. This month, I expand on that and discuss the microetch stage. I think microetch is the unsung hero of the surface finish line. It does not get the respect and, sometimes, the attention it deserves. The microetch sets the stage of creating a surface that will deliver the desired functional performance in end use.

The main role of the microetch is to create a uniform surface topography and micro-roughness to ready the copper for plating or organic coating. Achieving this desired surface roughness delivers the expected performance criteria such as solderability, wire bondability and corrosion resistance. The microtopography also aids in proper adhesion of the final plated metal or coating. It should be noted that the surface roughness created by the steps leading up to the final finish have the greatest effect on the copper roughness. They create the macro-roughness of the PCB surface. The microetch will then slightly alter that macrotopography. Close attention should be paid to the uniformity and degree of macroroughness prior to the final finish steps because there are instances where it cannot be overcome or smoothed enough by the final finish etch step. This includes, but is not limited to, deep scratches on the panel surface, but I believe we have covered this aspect previously.

Control of chemical concentration, etch depth and proper rinsing surrounding the microetch in the final finish line is critical to the ultimate surface finish performance. For thin layer coatings like OSP and immersion silver, the micro topography has a stronger effect on product reliability. It is less influential on performance properties of ENIG or immersion tin but critical to metal adhesion in those processes. In addition, the microetch could:

  • Undermine the garbage the cleaner could not remove.
  • Protect plating baths.
  • Remove oxides, which promotes uniform plating.

Considering the nature of etch chemistry, microetch rate can change significantly over a few hours. The etch is a dynamic chemistry, which means it is constantly changing as a function of time, temperature and copper growth. All promote breakdown of the active materials in the system. For this reason, proper analysis, maintenance and control of the etch rate are critical for this bath. As the active materials break down, the strength of the bath can diminish without proper control.
It is best practice to take an etch rate frequently on the same copper that is also used on your production work. Different copper plating chemistries and different copper styles will etch differently. For a final finish line, I recommend only using acid plating copper for the etch rate coupon, not copper-clad panels. Rolled copper foil does not etch at the same rate as electrolytic copper-plated parts. Consult the recommendations in your chemical supplier’s TDS and Operating Guides. When run out of specification toward the low end of concentration, the etch rate will decrease, resulting in low etch depth for the suggested dwell time. High concentrations can lead to high etch depth and undesired galvanic corrosion. An example of this is observed with selective OSP processing. When the microetch is not properly maintained, a staining can be observed on connected pads.

Historically, generic etch chemistry like peroxide or persulfate-based systems was used for all processes throughout the manufacture of a PCB. Though the backbone of the etch formulations has not changed much, proprietary additives have helped to overcome challenges and transform the etch to handle the new designs on the market today, and are now designed specifically to complement the surface finish.

Last, I cannot stress enough the importance of the rinse after microetch. Ever experienced metal peeling issues in the winter months? My guess is that you are using “ambient” rinsing. What a nondescript term, ambient. I am sure we all realize that the ambient temperature in Penang, Malaysia, is slightly different from Owego, New York, in January.

Let’s take a closer look: The average January temperature in Penang is 28°C (82°F), while Owego ranged this year from -12°C (10°F) to as high as 10.5°C (51°F). Now, I know the plant manager is going to say it never gets down to -12°C on the shop floor, but if the facility were shut over a cold weekend, it is far from warm on the plating floor come Monday morning.

The point is cold rinses do not rinse well. Why? Cold water does not solubilize salts as quickly as warm water, so if the rinse water is coming in cold, it is not properly removing etch salts from the copper surface. The chemistry and salts then get trapped in these areas and either pollute the subsequent plating baths or prevent the bath from plating properly in these localized areas, resulting in performance defects (Figure 1). Also, surface tension is temperature-dependent. It decreases with increases in temperature. Rinses will also get in and out of holes and plugged vias much better if the water is warm.



In closing, microetch should get the same attention as the plating and coating baths in the final finish line. Running the bath and the surrounding rinse water under proper conditions will prevent performance issues on finished goods.

Lenora Toscano is final finish product manager at MacDermid (macdermid.com); This e-mail address is being protected from spambots. You need JavaScript enabled to view it .

Last Updated on Monday, 01 April 2013 16:18
 

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