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Conformal Coating Defect Solutions with NPL, SMART Group
March 5, 2014 |Estimated reading time: 4 minutes
Conformal coating has provided benefits to the industry for many years, both in the high-reliability market sector and where products must deal with extreme environmental conditions. In recent years, due to changes in technology and miniaturization, coatings are being used in different industries including telecommunications, automotive, and consumer products.
NPL and SMART Group are creating a special feature area and free seminars sponsored by SMT Magazine at National Electronics Week (NEW) in the UK. This feature area will allow engineers and operators to have hands-on experience with different aspects of coating and cleaning plus education via a three-day seminar program. For further information, and to register for your free seminar spot at the NPL/SMART Group Conformal Coating & Cleaning Experience, April 8-10, 2014, click here.
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 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 optimization, 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. As part of the coating and cleaning experience we are also running a free pre-show webinar. For information and the book, click here.
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. A simple fluid test indicator, designed by Zestron, tests for activators and resin on the surface of joints either before or after a cleaning operation and prior to coating.
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 one for 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. It is always important comparing like for like.
To aid engineers conduct cost effective training sessions, two do-it-yourself training resources have been produced. Inspection posters and photo CD library allow staff to create their own training materials.
Conformal Coating and Cleaning Seminars
Tuesday, April 8:
- 10.30 Design Rules for Conformal Coating and Cleaning
- 11.30 Clean or No Clean for Conformal Coating Reliability
- 1.30 Selecting a Coating Based on Product and Environment
- 2.30 Reworking Conformal Coating and Component Damage
- 3.30 Process and Field Failures with Coatings
Wednesday, April 9:
- 10.30 Dip or Spray Coating Advantages and Disadvantages
- 11.30 IPC, ESA and NASA Inspection Requirements for Coating
- 1.30 In Process Quality Control for Conformal Coating PCBs
- 2.30 Cleaning Printed Board Assemblies for Coating
- 3.30 Confirming Component Compatibility with Cleaning Materials
Thursday, April 10:
- 10.30 In House or Contract Coating – Cost and Implementation
- 11.30 PCB Contamination Testing Options for Coated Boards
- 1.30 Test Methods for Conformal Coating Reliability
- 2.30 Conformal Coating Defects Causes and Cures
For more information and to reserve your spot, click here.