Publication date: 22 October 2009
Conformal coating has provided benefits to industry for many years, either in the high reliability market sector or where products have to deal with extreme environmental conditions. In recent years, due to changes in technology and miniaturization, coatings are being used in different industries like telecommunications, automotive and consumer products. Mobile phones and other professional handheld products have benefited from the use of selective coating for environmental and robustness. The following are examples of good and defect coating processes and some of the problems experienced in industry.
Coating overspray has been detected during inspection under UV light. The outer case of this connector shows evidence of the coating which may be due to incorrect programming of the robotic spray system. Although the connector could be masked this adds to the time and cost in manufacture.
Even if the engineer’s programming is correct, changes or fluctuation in process parameters may be the root cause of the problem. However, variations in the positioning of the product on the equipment may be a contributing factor to the contamination. Although using a standard blank board can be used for machine set-up and optimisation, an assembled board must be used for machine program confirmation.
The conformal coating applied in this example is excessive which has probably lead to the bubbles forming during cure. The excessive thickness allows the outer surface to dry and form a skin which does not allow further solvent in the bulk of the coating to evaporate allowing the bubbles to form.
If bubbles are only visible on the top surface of the Quad Flat Package (QFP) they should be considered a cosmetic defect, not requiring rework. Modifications should be made to process parameters rather than reworking the product. There is also some pull back of the coating on the edges of the component, possibly due to the thick deposit on the top of the device.
Coating on the board assembly should have had a straight edge, it is not clear why the uneven application line has happened in production. The coating was applied using an automatic spray system and previous example boards were satisfactory. Running a blank test board or test cards prior to coating fully populated boards should be standard practice before running a production batch.
Traditionally contamination on the surface of a board assembly is tested using a contamination measuring system which is explained on the new interactive part of the coating CD-ROM from the author. A simple fluid test indicator test designed by Zestron is available, it tests for activators and resin on the surface of joints either before or after a cleaning operation and prior to coating.
Basically the test fluid is placed on sample areas of the board assembly or on selected joints. The fluid must be left on the surface of the test feature for a fixed period of time for a reaction to take place. There are two different fluids, one for activators and resin systems. In the example a through hole termination produced by selective soldering has been tested for flux activators. The change in colour to a blue deposit shows a positive result indicating there are activators still present on the solder joint surface. Blue residues can also be seen around the joint on the solder mask. A company would need to establish their own criteria between this test method, cleanliness results, SIR reliability data and possibly product exposure to specific operating environments.
Satisfactory conformal coating on Plastic Leaded Chip Carrier PLCC) terminations under UV light. The image shows that the coating has covered all the joints surfaces and the areas between the terminations on the surface of the printed board.
Coating is satisfactory and is shown under a UV light source. There is good coverage on the component leads, component body and the surface of the board between the 0.050” pitch terminations.
Satisfactory dip coating of this SOIC is shown during inspection under UV light. The coating has completely covered the surface of the board, terminations and body of the component. If a spray coating process were used all the surfaces would be coated to an equal standard but there would not be coating filling the gap between the component termination and body. Its always important comparing like for like!!
To aid engineers conduct cost effective training sessions two do it yourself training resources have been produced in the UK. The inspection posters and photo CD library allow staff to create their own training materials
A new interactive CD-ROM Introduction to Conformal Coating Process Inspection and Defect Guide has been produced with the support from Gen3 Systems, Humiseal and SCH Technologies and features many of the common defects. The CD is available in the US from IPC, SMTA and ElectronicsCA in Canada.
Bob Willis is a process engineer working in the electronics industry, providing training, consultancy and product failure analysis. Bob also offers on site workshops on conventional and lead-free for customers. He will be running workshops at ITRI Merlin Circuits during October-November and at IPC MidWest and SMTA International in September details at www.bobwillis.co.uk Bob is happy to assist you set-up and optimise production lines for users and also provides conferences and workshops worldwide.
