It is well known in the electronics manufacturing industry that cleanliness of a Printed Wiring Board (PWB) is crucial to the assembly’s performance and reliability. Contamination of circuit boards can bring about severe degradation of insulation resistance and dielectric strength. Cleanliness of completed circuit boards is, therefore, of vital interest. Monitoring and quantifying the degree of cleanliness is necessary in order to ensure that the final cleanliness of an assembly is acceptable.
Harmful residues and contaminants are separated into two main categories: ionic and nonionic. Ionic residue can be described as residue that contains molecules or atoms which are conductive when in solution. With the addition of moisture ionic residues can disassociate into either negatively or positively charged species and increase the overall conductivity of the solution. Some of the most common sources of ionic residue include:
Nonionic residues are not conductive and are usually organic species that can remain on the PWB after board fabrication or assembly. These include:
While both ionic and nonionic contamination can impact the operation and reliability of the device on which they are present, the effects of ionic contamination is of greater interest to most PWB manufacturers. A higher figure of failures is associated with ionic contamination than its nonionic counterpart. Corrosion and dendrite growth are the two most common failure modes resulting from ionic contamination. Either can lead to shorts and opens in an electronic circuit, particularly in fine pitch applications or assemblies with high component packing density.
Nonionic (nonpolar) residue will lead to unwanted impedance because of its insulative properties. This is an issue particularly where plug-in contacts or connectors are utilized. In addition to acting as an unwanted insulator, non ion contamination on an assembly can cause poor adhesion of solder mask and conformal coating, physical interference with moving parts, encapsulation of ionic contaminants, and retention of foreign debris.
To learn more about cleanliness testing from Diamond MT, please contact us today!