February, 2013
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Copper Foil Thickness: Resolving Measurement Madness
Continued from previous page
Further, an acid copper electroplating operation does not precisely follow a theoretical model. True, it is a time vs. plating current density function, but the current density is not uniform over the 3-D to- pography of the printed circuit board. The plating will be thicker or thinner depending upon the ef- fective surface area exposed to the erosion anode (the source of the copper that is being plated). What all of this means is that to meet one re-
quirement, the fabricator may have to violate an- other. In the case of the fabrication drawing asking for one mil in the hole and 2-oz. finished copper, the fabricator must make a choice. If he chooses to use the ounces, he must either plate additional copper in the hole (1.3 mils or more) or he must start with a thicker foil — for ex- ample 2-oz. The thicker foil is very problematic given the circuit densities in today’s designs. The fabricator now will have a finished thickness of 0.0037-in. (3.7 mils/0.094mm), which severely limits the line widths and spaces that can be imaged, plated or covered with solder mask. In order for the fabricator to use
this approach, the PCB layout must have been performed with this process in mind. Unfortunately, there is seldom the needed communication between the fabricator and the PCB designer to allow this solution to be viable.
IPC Standards The IPC — Association Connect-
ing Electronics Industries®, is the rec- ognized standards body for the elec- tronics industry in terms of fabrica- tion and assembly of printed circuit boards. The IPC-4562 allows a foil tolerance of µ10 percent. This means the delivered copper foil could vary between 0.00121 and 0.00147-in. (0.0307 to 0.0373mm). Therefore, if the fabrication drawing references an IPC acceptability class (such as IPC class 2 in the IPC-A-600 Rev H Ac- ceptability of Printed Boards), and also uses ounces, the drawing has by default agreed to a thickness range. Therefore, should the fabrica-
tion drawing ask for two ounce fin- ished thickness and also IPC class 2, the customer is assumed to have agreed to accept a minimum of 0.0022-in. (0.0559mm) in finished copper thickness. The fabricator would be in total drawing compliance should they begin with one ounce foil and plate up the minimum copper (in- hole) thickness for IPC class 2 of 0.0008-in. (0.0203mm). This process has become the fabrication industry standard for addressing a request for 2-oz. finished copper.
Technical Purity There is an additional tolerance
that is not significant in a gross meas- urement, but should be noted for technical purity. The cause is the treatment of the foil, which is signifi- cant in laboratory analysis and foil performance. The copper foil supplied to a fabricator is not truly a flat sheet. In reality, it has typically three dis- tinct zones. There is a thin outer treatment, the base foil, and the sub- surface treatment. The two outer treatments are “rough” to allow ease of bonding or further processing. This is particularly true of the sub-surface. The sub-surface has what the indus- try calls, “a tooth”, which is a pyram- idal structure that increases surface area (for bonding) but reduces the
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copper mass in the cross sectional area. Depending upon where a measurement is taken, this irregu- larity can have up to 1 percent of tolerance in the gross measure of thickness. In the cross-sectional analysis of a finished
printed circuit board, the measurement is usually taken at a 50 percent point in the sub-surface treat- ment to the top of foil. This measurement location corresponds roughly to the valley point of the irreg- ular tooth in the sub-surface treatment.
Don’t Mix Oz and Mils l
The use of ounces for finished copper on outer layers should be avoided if the absolute cross sec-
“mils” should be avoided. l
In practice, the juxtaposition of ounces and
tional area of the trace or plane is critical. The finished copper of the plane or trace should be specified in a true thickness measure (thou-
sandths of an inch or micro meters) l
base foil for the inner layers. l
layers should be avoided In the case of sequential lamination because there is a surface plating on the
The use of ounces for finished copper on inner
The use of ounces for inner layer planes or traces, that are not sequentially laminated, is ac- ceptable as these usually a base foil that is erod- ed somewhat by the processing of the inner layer. Contact: DIVSYS International, LLC, 8110
Zionsville Road, Indianapolis, IN 46268 % 317-405-9427 fax: 317-663-0729 E-mail:
sales@divsys.com Web:
www.divsys.com r
Page 33
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