Recently, a customer contacted the EMPF and requested advice for replacing two wires on an RTV silicone conformal coated printed circuit board (PCB).

During lot production, two power wires were improperly installed and needed to be replaced (Figure 2-1). The customer reworked some of the PCBs which later failed in the field. On the failed boards, dendrites had grown under the conformal coating and between the solder joints of the reworked power wires. The customer requested help from the EMPF with improving their rework and cleaning method and to confirm the effectiveness of the method.

The PCB was conformal coated with a non-corrosive, one-part, fast moisture curing, RTV silicone elastomer (Dow Corning 3-1965). This coating offers good dielectric properties over a wide frequency range and resists high humidity and other harsh environments. The silicone can be applied by spraying, dipping, brushing, and flow coating. The coating in the rework area was carefully removed with a cotton tip swap and the application of a commercially available stripping agent such as Amtex-CCR or Envirosol.

Conformal coating is only effective at preventing dendritic growth if there are no ionic residues trapped under the coating. If residues are present under the coating, small amounts of moisture will penetrate the conformal coat, facilitate the migration of metallic ions, and affect PCB performance. Thus, all residues must be removed from the rework area prior to reapplying the conformal coating.

The EMPF reviewed the customer’s rework and cleaning process. An aluminum heat sink located next to the rework area presented a challenge to cleaning this PCB after rework. Since the heat sink could not easily be removed, and to reduce the possibility of capturing residues under the heat sink, the customer required that the PCBs be hand cleaned instead of cleaned with a batch cleaner. The original rework process called for RMA flux applied to a copper solder wick to remove the SnPb solder from the original solder joints. The replacement wires were then soldered with RMA flux and no-clean flux core SnPb wire. After installing the new wires, the rework area was hand cleaned with a brush and the conformal coating was reapplied.

The EMPF recommended using an RMA core solder wire instead of the no-clean core solder wire and recommended rinsing the rework area after hand cleaning. Mixing no-clean fluxes with other flux types is not recommended. No-clean fluxes contaminated with RMA fluxes cannot be easily brush-cleaned with DI water and isopropyl alcohol (IPA). If left behind, they do not cure properly and can absorb moisture. This enables the migration of ionic residues and the formation of dendritic growths in the presence of a voltage gradient. The recommended cleaning method for no-clean fluxes is the use of a saponifier at 60ºC and rinsing with plenty of low pressure steam and DI water.

A third recommendation was the application of a peelable mask, such as Chemtronics’ Chemask,® to the heat sink/board interface around the rework area (Figure 2-2). The mask creates a barrier to minimize pulling residues under the heat sink through gravity or capillary action. IPC rework methods (Class 3 guidelines for wire attach) were used to remove and replace the wires. During rework, care was taken to keep the PCB at an angle to encourage excess flux to flow away from the heat sink. Maintaining this angle is especially important while cleaning the rework area prior to reapplying the RTV conformal coating. The rework area was rinsed several times to remove ALL contaminants (Figure 2-3). Finally, the peelable mask was removed and the conformal coating was reapplied.

To confirm the effectiveness of cleaning method, local area ion chromatography (IC) was used per IPC-TM-650, method 2.3.28 using the C3 Localized Extraction Method and Dionex ICS 2000 ion chromatography (IC) system. This method can test the cleanliness of a dime size area (0.1 in2) instead of the whole PCB. The localized IC test results indicated that the weak organic acids (WOA), anions, and cations in the rework area were below the maximum EMPF recommended limits.

After reapplying the conformal coating, the EMPF subjected five reworked boards to 500 hours of temperature (85ºC), humidity (85%RH), and bias (12V, 1 amp) testing (THB). The goal of THB testing is to confirm that there were no deleterious effects of the cleaning and re-attach process. After testing, the reworked areas were microscopically examined with an Olympus SZX12 microscope, using 7X to 50X magnification. No residues, dendritic growths, or conductive anodic filaments (CAF) were observed in the rework area during optical microscopy (Figure 2-4). The lack of residues and the results of the THB test show there is minimal risk of leakage current and corrosion problems with the rework method developed by the EMPF and the customer.

In conclusion, ionic residues left under conformal coatings can lead to corrosion or leakage currents due to dendrite growth. Moist ionic residues are common causes of electronic opens and shorts. When reworking a conformal coated board, it is important to use compatible flux chemistries for all stages of the rework process and to clean and thoroughly rinse the rework area prior to reapplying the conformal coating. The EMPF offers various analytical techniques (THB, IC, ROSE, and FTIR) to determine the root cause of contaminant problems and to
evaluate the effects of rework processes on reliability. More information about these services can be found on the EMPF website, www.empf.org or by calling the EMPF technical staff at 610.362.1320.

References:
Munson, Terry. “Does Conformal Coating Stop Electrical Leakage Problems?” Circuitnet. Nov. 2004.

“Terry Munson.” Can You Clean a No-Clean Assembly?. 2008.
<http://www.residues.com/pdfs/Clean_No_Clean.pdf>.