Evaluating Manufacturability and Operational Costs for New Conformal Coating Processes Print E-mail
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Written by Jason Keeping   
Wednesday, 30 April 2008 19:00
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Evaluating Manufacturability and Operational Costs for New Conformal Coating Processes
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“Breaking down” the key process variables.

The original CC-Tango test vehicle1 was extremely useful in generating a database of qualification and reliability results for conformal coating application designs, and in understanding the interactions between their various process variables. However, the initial paper on the CC-Tango TV contained no upstream (masking/cleaning) or downstream (inspection/rework) process evaluations for their impact on the coating process. That is remedied in this article.

Given conformal coating equipment, processes and chemistries are in continuous flux, a process qualification tool was required to understand these changes in both coating materials and application equipment.

New conformal coatings, such as low-volatile organics (LOC) and UV-based materials have been marketed. Depending on the intended end-use, common reliability tests may include fungus resistance2, thermal shock3, hydrolytic stability4, dielectric withstanding voltage (DWV)5, flammability6, moisture and insulation resistance7, adhesion8 and salt-fog chamber testing, as well as mechanical product-level testing.

The CC-Tango TV is intended to generate baseline reliability data on conformal coating and to assess the impact of TV design features on the reliability of various DfM impacts. Although space restrictions limited the number of design feature variations included and tested, the TV included two variations of chip components to examine the effect of component locations on bubble (void) creation for reliability – a common process error.

A standard process for evaluating any new conformal coating materials or process alterations is required.

To further improve results achieved from the initial CC-Tango test vehicle evaluations, the various upstream/downstream processes were required to be tested, evaluated and understood for more complete overall process awareness.

The upstream/downstream processes assessed were cleaning operations, masking materials/processes and inspection/rework operations (Figure 1).

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With the upstream/downstream processes tested, a closer look at the actual conformal coating applications in both material and application types could be evaluated now in isolation, leading into a process that could be used for site enablement and improvements.

Upstream/Downstream Processes

Cleaning. The first upstream/downstream aspect evaluated was the cleanliness levels. The single most common deterrent to adequate conformal coating coverage and adhesion is surface contamination. In that, within the presence of ionic residues, oils, residual water and FOD (foreign object debris) on the board or component surfaces can result in corrosion, poor adhesion and future failure of the conformal coating. This can occur even when using low-residue fluxes/no-clean processes, especially if traces of flux residue are on the assembly because of manufacturing processing or improper cleaning. Cleanliness standards for evaluating these variations include IPC-SC-60, IPC-SA-61 and IPC-AC-62.9,10,11 This stated, the best method to minimize these potential issues is a thorough cleaning and subsequent drying process.

With time, testing and research, four specific variable branches, consisting of 11 variables, have been found to affect the output of a cleaning process. These four branches can be labeled: Assembly Materials, Chemistries, Components and Cleaning Process. The variables within this section have many subsets to explore, including various assembly materials, chemistries, components and potential cleaning processes. However, this work’s main intent is to explore conformal coating operations; thus, a simple subset of the various global manufacturing processes employed were based against either a standard cleaning process or not being cleaned.

The interesting finding was that, regardless of cleaning process used, for all tests completed, the washed samples produced less conformal coating coverage and adhesion issues.

Masking. Masking is another process that if not performed correctly can leave residues on the assembly that can affect the conformal coating coverage or adhesion. Masking materials include the following forms: silicone boots, covers, peelable masking, and masking tapes.

It is important to validate compatibility between masking and coating materials. In that, within the various masking materials, some contain a substance that is incompatible with coating materials and may even inhibit coating curing.

With time, testing and research, three specific material branches, consisting of eight material types, have been found to affect the masking process. These three branches can be identified as protective devices, encapsulants and tapes (Figure 2).

Image

The variables within this section have many subsets that can be explored, including the vendors and materials for each branch variable. Again, based on this work’s main intent, a simple subset of the masking tapes was evaluated (Figure 3).

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Requirements used to evaluate the masking materials were:

  • Ease of placement/removal.
  • Filtration of conformal coating.
  • Delaminating/adhesion loss of coating when removed.
  • Paste residue after removal.
  • Dewetting around the tape material.

A key point that should be noted from this evaluation was all masking materials were non-ESD compliant and required the use of an ESD ionizer during both removal and application. Overall, with ionizers, no samples exceeded ESD limits.

Inspection/rework. The third upstream/downstream aspect evaluated was the inspection/rework processes used in the modifications and evaluations for conformal coating.

Compared to cleaning and masking upstream processes, inspection/rework is considered an inline and downstream process to the conformal coating process. This said, a common element to both the inspection and rework operations is lighting, playing a fundamental role in both detection and evaluation as to where the conformal coating is, where it is not and the material quality.

With time, testing and research, various light sources were found with wavelengths between 254-365 nm, with an optimal wavelength of 365 nm that produced both a quality source and safe inspection for operations, in both regular and darkened lighting.

Test Vehicle Design

For a typical conformal coating reliability test, the test vehicle design requirements are governed by IPC-CC-830.12 The specification was not, however, written with end-user inspection requirements in mind. Regardless, many of the primary considerations do not change, and the specification remains a useful starting point in designing TVs and test plans for evaluating conformal coating materials in a manufacturing process.

The CC-Tango TV layout was based heavily on the concepts to produce a one-shot process to establish a consistent global application/inspection method for existing conformal coating customer products, and new accounts and sites.

The CC-Tango TV was sourced in a standard 0.062" thickness with six metal layers. A total of 137 components are included within the TV to assess the various topologies that a conformal coating process would encounter and process defects that could arise in a process.

With the listed components, the CC-Tango TV was divided into five sections covering the following test requirements:

  • Connector wicking and daisy-chain components.
  • Various pitch, lead formats and surface orientations.
  • Discrete spacing and configuration for bubble (void).
  • PCB tooling hole wicking for material penetration.
  • Nozzle placement accuracy and definition.

Figure 4 shows these test strategies. CC-Tango design overview and descriptions are in the following sections of this article. For reference to the conformal coating development tools within each section, see Keeping.1

Image

Section 1: The key aspects of this section for cleanliness testing are the connectors and BGA components. In that, the connectors chosen actually are supported off the assembly by 2.28 mm within this model and provide an area for post-soldering residues to build after wave soldering.

However, for the BGA components, post-soldering residues are a prime concern for cleaning, with inspection under these types of devices difficult or impossible. These are a couple reasons why many of these devices are now being underfilled prior to conformal coating.

Section 2: The key aspects of this section for cleanliness testing are the same physical variations in surfaces and component types that were selected. However, not for the material application requirement, but for flux entrapment during either SMT reflow or wave soldering (Figure 5).

Image

Cleanliness for these components is critical, prior to being coated. In that, these components can vary in cost and removal due to contamination, depending on the coating material, and could damage the assembly in the process.

Section 3: The key aspect of this section was split between cleanliness and masking material testing. For cleanliness, with the various discretes and configurations, a lower standoff was provided for flux entrapment to be evaluated, with a FR-4 tab exposed for masking material and coating testing (Figure 6).

Image

Section 4: With regard to the upstream/downstream processes, only a small impact on this section was included for cleanliness testing: the entrapment for FOD within the various tooling/via holes or flux entrapment runoff during SMT reflow or wave soldering (Figure 7).

Image

Section 5: Covers nozzle placement accuracy and edge definition for control awareness testing for material and rotation (Figure 8). The purpose of this section was to investigate the placement accuracy and edge definition control of the process being evaluated. Poor results in this section should be avoided, since inadequate control of a film pattern could lead to coating migration into an undesired location. This can be caused from several variables, such as start/stop, overlap, and even spray atomization. If a start/stop is not corrected, a process defect described as a dog-bone can occur. The defect of this issue is a controlled pattern has a process limitation on closeness to a keep-out; however, if a dog-bone occurs, material pooling can occur at the end and lead to coating migration into the keep-out location. Sample defects are shown in Appendix 6 and 7. With regard to the upstream/downstream processes, this section was designed for application testing with minimum impact on either cleanliness or masking testing.


Conformal Coating Process

With the knowledge of the upstream/downstream processes, we can look into the process requirements that must be completed to maximize the coating application results and provide the highest return for the manufacturer’s and end-customer’s requirements.

Material. The first and primary process requirement that must be understood is the coating material selection that is completed. In that, each class of material has both advantages and disadvantages. Several common types of coatings are generically described as acrylic resin (AR), urethane resin (UR), epoxy resin (ER) and silicone resin (SR). (For briefings on their advantages and disadvantages, see the online version of this article).


Last Updated on Friday, 18 July 2008 09:34
 

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