About Pinholes and Fractures

Some products or applications require a material with high barrier properties in regard to moisture and oxygen. That is to say that the materials essentially prevent the intrusion of moisture and oxygen. Such materials are primarily metal foils or metal-plastic foil laminates. Due to cost constraints, the foils are made as thin as possible. Because the materials are thin, defects and damage can lead to perforations.

Pinholes are perforations that are generally less than 100 microns in size and are present in virgin material from the factories. Pinholes can be caused by contaminates present in the foil's molten state, or from rolling debris and fines during foil production. Virgin thin foils are expected to have pinholes. The table below shows allowable pinholes for different gauges of aluminum foils from one foil manufacturer.

Maximum Allowable Pinhole Count in One Square Meter 

Foil Gauge	Foil Caliper, μ	Average	Maximum
28.5	7	423	1584
35	9	211	1056
50	13	85	528
75	18	21	106
100	25	0	0

(From Alcan/Pechiney General Raw Material Specification for Aluminum Foil # 14000G )

Damage from handling or fabricating are the primary sources of other types of foil perforations. These perforations are typically larger than 100 microns. Examples are fractures, punctures and tears. Probably the most common occurrence of these types of perforations is in the fabrication of formed cavities. That is because the already thin foils are thinned even further to form the cavities. Variations in the foil or process can result in perforations.

Exacerbating the situation, if a pinhole is present in the forming area before the cavity is formed, it increases the probability of a perforation when the material is formed. 

When considering perforation inspection for a particular application, an important question arises - what size pinhole needs to be detected?  That depends on the application. The end user must make the determination. Only the end user knows the exposure to oxygen and moisture that his product can tolerate. In the pharmaceutical industry 25, 50 and 100 µm are common detection levels.

Why not use as small a pinhole size detection as technology can provide? The problem with that is that thin foils are expected to have some pinholes. Given that, if the pinhole size is tolerable by the product, it is a waste of time, material and money to reject it.