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www.us-tech.com Resolving Measurement Madness Copper Foil Thickness: By Stanley L Bentley, P.E., Senior Technical Advisor, DIVSYS International, LLC, Indianapolis, IN
each other. To understand the reason the confusion has persisted, it’s important to first understand that there are three stake holders in the commerce chain, and they do not necessarily speak the same language or communicate with each other. The stakeholders are: The Foil supplier, the Fabricator, and the End User. The units of confusion are ounces and mils (1 mil is 0.001-in. or 0.0254mm).
T
Ounces: The Foil Supplier This is a very antiquated apothecary measure
that has somehow survived in the PCB industry. The designation “ounces of copper” must be inter- preted. One ounce by English measure of weight and having a density of 5.14oz/in3 (8.9g/cm3) is uni- formly spread over 1 square foot of area (144-in.2). In the case of copper, this process will yield a fin- ished thickness of 0.00135-in. (0.00343mm). The industry now “rounds” this up to 0.0014-in. (0.03556mm) in “print”, but not in fact. This meas- ure in the “tribal knowledge” of the industry is often further rounded up to 0.0015-in. (0.0381mm). Depending upon whom is asked (Copper sup-
plier, Print medium fabricator, or User), the defi- nition of “One Ounce” can be three figures. Howev- er, in terms of what is actually delivered, it is al- ways from the measure of the copper foil supplier.
MILs: The Fabricator From the perspective of the Fabricator of a
Printed Circuit Board, measurements are made in “Mils” or thousandths of an inch when referring to thickness. The fabricator purchases laminate with copper foil applied, but does not have any control over the unit of measure in which the foil is sup- plied. The fabricator then plates up or chemically
Microscopic inspection helps to monitor plating.
erodes the foil that the company has purchased (in ounces). The fabricator will always specify thick- ness of copper in Mils. The most common use of this term is to define the thickness of the copper
he copper foil on the printed circuit board has two very confusing units of measure. These units are frequently mixed and confused with
plated in the finished hole of the printed circuit board. Plated copper thickness is often referred to as “1 Mil in the hole”. This means the fabricator has plated up 0.001-in. (0.00254mm) of copper in the hole and on any external copper foil that is ex- posed during the electroplating process.
End User Specs The juxtaposition of “ounces” and “mils” is
common on fabrication drawings. The end user often specifies the finished thickness of the copper in ounces instead of mils. For example, a customer may ask for 2 ounces of finished copper. This inter- pretation is not simple for the fabricator to deter- mine or deliver. The fabricator will have conflict- ing requirements if the customer also asks for “one mil in the hole”. If the fabricator begins with one ounce foil and plates up one mil, the external cop- per will have a theoretical thickness of 0.00235-in. (0.0597mm). Two ounce “foil” would have a theo- retical thickness of 0.0027-in. (0.0686mm), a differ- ence of 0.00035-in. (0.00889mm). In practice, the differential is even greater. This differential is often a cause for confusion and confrontation.
More Fabricator Problems The fabricator has an additional issue not ad-
dressed by theoretical measures. The company must deal with manufacturing tolerances and process losses. The fabricator must clean and “roughen” the copper surface of the foil that has been purchased. If this is not done, then it becomes impossible to apply additional plating with any de- gree of reliability or uniformity. These process losses are typically 100 micro-inches per major op- eration. Different types of boards have different “major operations” and therefore, more or less process loss of thickness due to chemical action.
Continued on next page
February, 2013
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