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dsi is sought out by many customers specifically for our high cleanliness standards. Cleanliness is a plant-wide commitment extending to both our printed circuit board and our electronic assembly operations. Cleanliness entails far more than just a final board wash. It requires a combination of cleaning techniques in almost every process throughout the PCB fabrication cycle and carries through to assembly with proper handling procedures and additional cleanliness systems. As shown below, dsi's standard for cleanliness is even more rigorous than prevailing industry standards.

dsi 1 microgram/cm2 Total Ionics IPC Standard 10 micrograms/in2 Total Ionics Military Requirements 10 micrograms/in2 Total Ionics

Why is PCB cleanliness important? Concern for field failures is a strong motivator. Designers must consider the materials used, the board design, and the operating environment in order to establish custom cleanliness specifications to meet industry standards and specific product application requirements.

The Effect of Cleanliness and PCB Design

The design of a PCB and the environment where it is used has as much, if not more, to do with electromigration failure as does the manufacturing process. For example, if you have two boards with 5.0 micrograms per square inch of NaCl, one has 10 mil traces and 30 mil spaces while the other has 5 mil lines and 5 mil spaces, the latter is most likely to experience failure. On the other hand, if you take two identical boards and use one in a hot, humid environment and use the other in a cool dry environment, the latter would probably not experience failures.

High Power and Dendrite Growth High power/current applications are more at risk with an unclean board since the high power yields dendrite growth at a faster rate due to the high delta in electrical potentials. High power/current creates a greater electrical potential between the traces. This ultimately allows for a higher failure rate and a smaller process window if contaminates are on the surface of the board or trapped in the mask. Think of this situation as being like a high voltage panel zapping someone standing in a puddle of water four feet away. Several conditions must be present for that to happen. The humidity must be high so the air can act as a dielectric, the voltage must be very high so the grounding system is saturated, and the person must be standing in a puddle of water. If you remove any one of these conditions, the person is safe!

Power Level and Metal Migration Higher trace densities increase the risk of electromigration. This is particularly true as designers push the recommended limits for spacing between traces while trying to fit circuitry into ever smaller packages. The higher power levels can reduce the process window. Smaller traces reduce the space the dendrites must grow across and increase the potential for failure.

Operating Environment and Corrosion Harsher assembly environments increase the potential for corrosive residue. Assemblers are being asked to use a wider variety of materials than ever before. Some of these materials perform masking or structural functions, which means the initial surface cleanliness is more critical since cleaning after these applications is difficult. Typically, matte soldermask presents the greatest challenge to maintaining cleanliness. It is porous and traps much contamination during processing. It is difficult to get the boards clean after the application of the soldermask because the soldermask cross-links during the exposure/heat cycles and literally encases ionics into the mask, lowering the resistivity of the surface. Although there is no definitive proof that certain types of laminates are more prone to contamination, any paper-based laminate holds moisture and moisture reduces surface resistance.

Ionic vs. Organic Contamination

What is the difference between total ionic contamination and organic contamination? Total ionics is a summation of all the ion species present -- sodium chloride, bromide, fluoride, sulfate, phosphate, weak organic acids, nitrate, and methane sulfonic acids (MSA). Organic contamination is a glycol-based contaminate that is hydrophilic, meaning it absorbs and holds moisture. Organic contamination can come from processing oils, poor rinsing and human handling. It lowers surface resistance by holding moisture.

Why is sodium chloride (NaCl) contamination so important? Sodium chloride (NaCl) has a very high potential charge. NaCl also disassociates well in a liquid medium. With the ability to dissolve in moisture and the high ion charges present in both Na+ and Cl-, the potential for failure is optimized.

What are the limitations of SIR testing? Surface insulation resistance (SIR) testing can detect the presence of either organic or ionic contamination, but it cannot pinpoint the type of contamination nor the degree of contamination. SIR testing results can only show whether the surface resistance is low. The presence of moisture (organics) or ionics allows current to flow across the laminate (soldermask) surface, lowering the surface resistance. Since electrical signals always follow the path of least resistance, this can contribute to failures.

Configurations and Techniques for Cleaning

Fluxes - PCB manufacturers concerned with cleanliness use only qualified and accepted fluxes in the manufacturing process. HASL (Hot Air Solder Level) fluxes contain amine hydro-bromide flux activators in a polyglycol carrier. If the soldermask is not permitted to cure completely, bromide residue can penetrate the porous surface and cause corrosive damage. The water soluble flux often used in the HASL process also increases levels of weak organic acid and chloride residues. To prevent the accompanying electrochemical failure, manufacturers must closely monitor chemical levels.

On bare boards, dsi recommends that water soluble flux chloride levels should be below 3.0 micrograms per square inch (msup>2).

Temperature - Temperature levels throughout the HASL process also affect cleaning. To increase the dissolution rate of HASL flux residues, the rinse water temperature must be kept above 140°F. Depending on the thermal threshold of the mask and laminate materials, manufacturers must maintain temperatures that allow cleaning materials to penetrate the surface completely. The temperature is frequently increased above 140°F to the highest tolerable level for deeper cleaning and surface pore expansion. In general, higher cleaning and rinsing temperatures produce lower contamination levels.

Brush Systems - Brush systems also improve board cleanliness. Oscillating approximately 200 times per minute, the brush bristles furrow into the HASL flux layer. The furrowing action of the bristles serves to disrupt the surface tension of the HASL flux residue, increasing the dissolution rate of the flux residue and improving the effectiveness of the cleaning process.

Deionized Water - Deionized (DI) water cleans more effectively than tap water. Deionized water typically has a higher resistive value and lower contaminant content levels than reverse osmosis treated water. Using highly resistive DI water in the final rinse stage lowers the amount of ionic residue that can potentially dry on the panel's surface. Manufacturers often include the use of a saponifier to ensure complete residue removal.

Ionic Test Methods

Resistivity of Solvent Extract (ROSE) Test Method IPC-TM-650 2.3.25 The ROSE test method is used as a process control tool to detect the presence of bulk ionics. The IPC upper limit is set at 10.0 min2. This test method provides no evidence of a correlation value with modified ROSE testing or ion chromatography. This test is performed using an ionograph or similar style ionics testing unit that detects total ionic contamination, but does not identify specific ions present.

Modified Resistivity of Solvent Extract (Modified ROSE) The modified ROSE test method involves a thermal extraction. The PCB is exposed in a solvent solution at a predetermined temperature for a specified time period. This process draws the ions present on the PCB into the solvent solution. The solution is tested using an ionograph-style test unit. The results are reported as bulk ions present on the PCB per square inch.

Ion Chromatography IPC-TM-650 2.3.28 This test method involves a thermal extraction similar to the modified ROSE test. After thermal extraction, the solution is tested using various standards in an ion chromatograph test unit. The results indicate the individual ionic species present and the level of each ion species per square inch.

Correlation Between Test Methods The correlation factors between test methods are as follows (with 1.0 being perfect correlation, 0 being no correlation):

ROSE method to modified ROSE method ROSE method to ion chromatography Modified ROSE to ion chromatography 0.18 0.21 0.94

dsi has found that the best process control method is modified ROSE testing verified with ion chromatography. ROSE testing is used only to detect bulk ionics prior to soldermask application. (See the correlation data below.)

Despite a great deal of study and debate, there is no industry-accepted correlation between contaminate levels and field failures. For this reason, IPC has been reluctant to change the acceptable limit from 10 micrograms per square inch (mg2/in) total ionics to a lower number.

Correlation Data Note: For comparison, the ion chromatography data is totaled rather than listed for each specific ion species