Publication date: 03 July 2008
High growth electronic products require performance on demand and miniaturization accelerating the need for thinner and highly dense circuitry. A common threat to the production of highly dense electronic assemblies is the growth of dendrites and intermetallics that ultimately cause short circuits.11 Literature on reliability risks of electric circuit assembly’s reports that this phenomenon appears to occur more frequently with lead-free electronics.11
The speed of electronic interconnection innovations outpace testing, regulatory, and standards bodies serving the industry.5 Miniaturization is constantly imposing new criteria and challenges on the soldering materials.7 With decreasing size of pad, through-hole, microvia, and pitch, the make up of substrates and components becomes more delicate, which increases the vulnerability of solder joints7.
Increased functionality and smaller devices represent significant drivers in innovative circuit designs7. Manufacturing complexity increases with the continuous drive for smaller lighter and more advanced features. In order to pack more circuits into a smaller device, everything within the device has to shrink in size.7 As packages continue to shrink, the convergence of circuit board and advanced packaging drive designers and component engineers to develop board level features that improve reliability.
Contamination can severally diminish the ability to resist shorting of leads or traces2. Pre and post-assembly ionic and non-ionic contaminants are considered harmful residues to the electronics manufacturing process. While both ionic and non-ionic contamination can impact the operation and reliability of the device on which they are present, the effects of ionic contamination are of greater interest to assemblers. A higher number of failures are associated with ionic contamination than its non-ionic counterpart.2
The problem is that existing IPC test methods cannot account for all process issues within the manufacturing process to predetermine the reliability of an assembly. The IPC standards provide an empirical measure for qualifying materials and processes, but do not account for variances that may occur in the manufacturing process. Localized contamination might not raise the contamination level of the overall assembly but may have a detrimental localized effect.
Lead-free is a disruptive change that requires manufacturing process change in a fundamental element of an electronics assembly – the connections between components15.
Higher I/O counts, progressive miniaturization, more reliable performance, and other demands are being impeded by circuit board cleanliness effects. The relevance of clean lead-free assemblies must be considered to mitigate stray leakage and electrochemical migration reliability risks.13
The change to lead-free manufacturing urges tighter process control11. Whereas previously an engineer could tune the process by increasing flux activity, changing conveyor speed, or increasing preheat ad hoc, high-tin/high melting point alloys are a different matter. Slight changes from a well defined tin-lead process can result in an increase level of board failures11.
A tightened lead-free SAC process window increases cleaning relevance. Boards commonly soldered with a no-clean process no longer can be tolerated11. If the flux oxidizes or chars during the soldering process, the board will experience high levels of ionic contaminants.
During high lead-free processing temperatures, the board laminate expands with the risk of absorption of contaminants in the mask. Such risks can and does cause electrochemical reactions and reliability problems11.
Miniaturization: Miniaturization imposes a great challenge on the chemistry of fluxes, due to the increasing amount of oxides and requirements for no-clean lead-free applications7. The size of the component and the conductor spacing generates more heat during operation6. Problems arise from boards with greater mounting density resulting in electrochemical reactions, metal migration, and reduction of surface resistance6.
Miniaturization often results in reduced wetting7. The oxide thickness does not decrease in proportion to pad size, thus consequently results in poorer wetting. A reduced joint size is also more prone to void formation, mainly due to a greater difficulty in wetting. Decrease in discrete component size inevitably results in an increase in vulnerability toward tombstoning, skewing, or billboarding7.
A smaller solder joint will be depleted by corrosion sooner than a larger joint7. Oxidation of fine solder powder increases with reduced particle size due to the increasing exposure surface area of powder. Changing from SnPb solders to lead-free solders further worsens the problem, since lead-free solders are more prone to corrode than SnPb solders7.
The barrier to the ingress of materials that can cause corrosion is reduced as package sizes decrease9. Deposition of contaminants can occur under wet or dry conditions. At 60% relative humidity, a layer of water two to four molecules thick can form.
Along with the water are contaminants, and even in this very thin layer chemical reactions can occur. When the relative humidity reaches 80%, the water layer is from five to 20 molecules thick, and ions can flow freely on the surface. At this point moisture and acids can form a corrosive cell9.
Condensation of water and contaminants onto the surface of an IC package occurs because of surface tension when the temperature passes through the dew point9. Changing conditions may cause the water to evaporate, but the contaminants are left behind9. In a humid environment and in the presence of electrical bias, excessive ionic contaminants on an assembly can cause problems. The issue is increased from tight board traces, board density and higher voltage.
At constant voltage, the electrical field between the conductors rises inversely with the conductor spacing6. As a result, electrochemical migration and the formation of dendrites are more susceptible from narrow conductor spacing6. The tolerance of electronic products toward dendrite formation diminishes rapidly7. Shorts between traces are due to electrolytic dendrite growth, erosion of conductors, or loss of insulation resistance11.
Lead-Free Flux Compositions: Compatibility of lead-free alloys with a variety of flux chemistries is considered essential, and is determined for performance in handling ability, including shelf life and tack time, and soldering capability, including solder balling, wetting, and solder joint appearance1. In general, SAC lead-free alloys exhibit a poor wetting ability. More aggressive flux chemistries are needed in order to achieve wetting comparable with eutectic tin-lead alloys3.
Solderability is a measure of the ease with which a joint substrate can be wetted by molten solder and it continues to an important consideration in all soldering processes, particularly as the industry moves to lead-free15.
Flux is necessary to remove surface oxides from PCBs and components to promote correct alloy formation and to act as a heat transfer medium to ensure correct soldering temperatures. The cleaner the solder surface the less activator is needed to solder the component16.
Lee (2008) cited desirable characteristics of lead-free no-clean flux compositions:
1. Reduced volatility;
2. Halide-free;
3. Greater fluxing capacity;
4. Higher residue resistivity;
5. More resistant to oxidation and charring;
6. More efficient oxidation barrier;
7. Lower activation temperature;
8. Slower wetting speed when solder begins to melt;
9. Less spattering,;
10. Higher probe penetratability;
11. Capability of inducing nucleation of solder upon cooling; and
12. Greater resistance to slump7.
With a no-clean process, the objective is for the residue to form a hard barrier that seals the circuit from exposed ionic contamination. For high reliability product suppliers no-clean flux technology introduces a difficult challenge. While manufacturers did not want to clean assemblies, they couldn’t risk exceeding the prescribed contamination threshold.
Flux compositions designed for lead-free consist of multiple polymer species and property modifying additives16. The additives affect the mobility of the system, solvent retention properties, long and short term dielectric properties, and thermal behavior.
The key to maintaining all desired product attributes, as well as maximizing topside fillet performance, lies in a thorough understanding of the interactions between these polymers and certain properties of the modifying additives16.
The science of flux compositions for eutectic Sn/Pb is well understand and mastered16. At higher soldering temperatures required for lead-free SAC alloys, the organic solvent must be thermally stable to reduce the level of activator needed to solder the component.
One critical problem is the lack of thermal stability, thus requiring more active flux compositions. This leaves a residue that is not longer fits the no-clean categorization16.
As soldering temperatures rise, the flux materials undergo changes in their physical and chemical properties such as the evaporation of volatile fractions, surface energy, and melt viscosity16. The consequence for the solder flux is early displacement by the scrubbing action of the solder wave, and ultimately the thermal breakdown of the material.
This results in loss of the flux functionality as a protective blanket, and the loss of an insulating film over the liquid solder when it wicks up the barrel of the via or though-hole. The latter result, in conjunction with the larger change in lead-free soldering temperatures between the bottom and the top side of the printed circuit assembly passing through the wave, results in early solidification before the liquid solder is able to wick up the barrel and wet the top side of the pad.16
In wave soldering operations, the activator systems must provide the ability for the molten solder to smoothly and reliably separate from the non-metallic when the board exits the wave16. The solder flux therefore must exhibit both a detergency and a highly controlled surface energy during the time that it is in contact with the solder wave.
It must rapidly spread over the surface, displace the contaminants, and build a uniform monomolecular film which presents the proper surface energy profile to the wave. This done using a flux composition requires surface active materials whose characteristics have been optimized for the acidic environment of the solder flux16.
For the monomolecular flux film to be stable, it must wet and be absorbed on the surface in a stable configuration during the high temperature excursions through the solder wave16. A combination of several surfactants is required to control both the surface tension and interfacial tension at the solid-liquid interface.
The flux chemistry must improve wetting on non-metallic surfaces, lower surface tension, and penetrate into the subsurface of the solder resist. Such a system needs to control the droplet size of the flux upon spraying, which is essential for controlled distribution of the flux and sufficient capillary activity in vias and through-holes. Such systems will impart more residues, foaming, reduced SIR and the need to clean16.
Electrical Failure Mechanisms: Electronic assemblers use surface insulation resistance (SIR) testing when adapting to changes in processes, materials, and manufacturing equipment. SIR is defined as electrical resistance between two electrical conductors2. SIR can be thought of as a systems ability to resist surface shorting or leads or traces. Performance of SIR test samples are directly related to cleanliness2.
SIR testing evaluates the propensity for assembly failure caused by shorts or current leakage between metal conductors2. These failures can be induced by material interactions, inadequate process control, or poor material performance. SIR is an electrical test that measures a change in current over time and is typically performed at elevated temperatures and humidity levels. Sheet resistance, bulk conductivity, and electrolytic contamination leakage are all factors that affect the insulation resistance2.
SIR measures an aggregate of different resistance of both contaminated and non-contaminated values in parallel4. The growth of dendrites or the presence of conductive solutions between the conductors of the patterns affects the resistance between them. Higher SIR values are the result of cleaner boards that do not form dendrites. Lower values are a result of the presence of conductive dendrites or salts2.
Electrochemical Migration: Electrochemical migration is an occurrence of a conductive metal bridge forming between conductors when they are subjected to a DC voltage bias5.
Metal conductors grow from a positively charged conductor (cathode) to a negatively charged adjacent conductor (anode) creating a short circuit between the conductors. The growth takes the tree-like form of a dendrite2. Dendrites form on any surface as long as there are residual ions, electrical bias, and condensing environments5.
Electrochemical migration consists of metals plating in reverse8. An ionic contaminant combines with water, generally forming a localized source of acid, which dissolves metal ions. Under the influence of the electrical potential, the metal ions move across the intervening space, plating out on the laminate as it goes, forming a metal filament8.
Electrolytic corrosion and electrical leakage have three elements that must be present:
1. an electrical bias or potential between two points;
2. the presence of liquid water or water vapor; and
3. ionic contamination8. Low voltage has less risk than high voltage. A more contaminated assembly has greater risk than a clean assembly.8
The added presence of moisture can cause ionic residues to disassociate into either negatively or positively charged species and create conductive solutions, known as electrolytic solutions2.
Chloride and bromide, commonly found in fluxes and PCB substrates, are two of the most common dendrite forming substances. Metallic salts from copper based metals, which conduct electricity and create shorts across the leads or traces, can also be formed in the process.2
Electrochemical migration provides a measure of contaminants and their affect on assembly reliability4. Metals subjected to humidity and electrical bias form dendrite growth in the presence of corrosive electrolyte. Conducting ions within the electrolyte may derive from corrosion or the metal conductors, or from improperly cleaned circuit board substrates.
Contamination may derive from the manufacturing of the bare PCB, subsequent handling, or from the application of corrosive fluxes without adequate cleaning. Studies have determined that failure due to electrochemical migration is based primarily on the cleanliness of the parts.5
Electronic assemblies using silver in their construction (immersion silver finish/SAC alloys) when operated at high voltage/energy levels creates a path for dendrite growth in the presence of moisture5. Failure of silver was experienced in the 1950’s and 1960’s when placed in uncontrolled environments with significant extraneous ionic contaminants.
In humid environments, ionic contaminants interacted under bias to form metallic dendrites5. Among common metals, silver is the most electro-active, meaning that it plates or de-plates the easiest8. In the presence of silver ions, less ionic contamination, less humidity, and less bias are needed to start the dendritic growth.
The soluble ions migrate towards the opposite polarity as the supply of electrons traveled through condensed water to reduce the ions into metal dendrites14.
The greater vulnerability of lead-free solders toward corrosion is attributed to galvanic corrosion induced by the presence of silver8.
Small dendrites may form a bridge but may spark away much like a fuse8. If the contamination is more pronounced, the area will go through a series of spark events, often charring the surface of the board (Figure 1).
As carbon is electrically conductive, eventually leakage occurs. Even without the carbon, if an electrolytic solution is present, current can still flow.8
Ionic residues surrounding metal leads will begin electrochemical migration when voltage is applied to the circuit in the presence of moisture.
Electrochemical migration in turn can result in accelerated corrosion, leakage current, dendritic growth, cross talk, and other anomalies (Figure 2).
The unpredictable chemical reactions that occur at sites where contaminants and moisture accumulate can corrode both metal and plastic elements – wires, wire bonds, and lead fingers9.
Even before a wire or a lead finger is corroded through, corrosion alters its conductivity.
Conductive Anodic Filament (CAF): Conductive anodic filament formation involves a growth of conductive metal salts, usually along capillary-like fracture within PCB substrate materials5.
CAF is the formation of conductive copper salt filament, starting from anode and growing toward cathode, along the delaminated interface between resin binder and glass fiber7.
Often CAF is initiated by cracks formed between resin and fiber during mechanical through-hole drilling process, and is aggravated by delamination caused by high soldering temperatures (Figure 3).
Even in the absence of mechanical drilling, such as substrates with microvia in pad, the CAF symptom can still be observed, and CAF is a concern within industry at via-via spacing less than 15 to 20 mils.
With reduction in pitch dimension, the chances of circuit short caused by CAF will increase7.
In highly dense circuit designs through-hole pitch becomes narrower from year to year and it becomes very important to maintain reliability. In the case of FR-4 CAF is becoming a serious problem10.
CAF is the result of Cu migration occurring through the glass fabric for high density PCBs. At first copper dissolves at the anode side and becomes a copper ion.
Then copper ions move thought the glass fabric, and finally copper ions reach the cathode side and deposit to form a dendrite10.
The CAF phenomenon is influenced by base material properties and processing of the PCB10. Laminate properties, drilling of small holes, and close plated copper traces accelerate CAF when moisture and ion contaminants are present. Organic laminates are prone to loss of insulation resistance due to conductive filament formation under accelerated conditions of temperature humidity and bias.
A loss of insulation resistance is developed between various configurations including plated-through-hole to a plated through-hole, and plated-through-hole to metallization line on the surface or inner layer of the laminate12.
The first step is the degradation of epoxy resin/glass fiber bond in the laminate leading to path formation12. The second step is the electrochemical process involving metal migration. Once the conductive filament formation path is formed, a short circuit often occurs with sufficient current to break the continuous filament that caused the insulation resistance. Metal ions can start to migrate once again when the electrolyte path reappears. The insulation resistance loss due to conductive filament formation is hypothesized as a two-step process12.
The reduction of spacing between conductors increases the time to failure due to conductive filament formation12. Larger spacing means a longer path must be formed between the electrodes. The humidity level must be high enough to accumulate sufficient moisture and form a continuous path.
Tighter spacing creates the effect of higher potential gradient – that is, providing a higher driving force for ions to migrate. The effect of spacing on time-to-failure correlates with line space with the tighter spacing resulting in less time to failure12.
Higher voltage bias can lead to quicker failures due to conductive filament formation12. The test pairs (FR-4) laminate subjected to 300 volts of DC failed later than the ones subjected to 800 volts DC. For 20 mil spacing, the test pair had a life of 450 hours under a DC bias of 300 volts compared with a life of 100 hours under a DC voltage of 800 volts.
The bias between electrodes can affect filament formation from the velocity of an ion in an electrical field and by the voltage gradient. When a higher voltage gradient is applied, ions move faster in the electrolytic medium, speeding up the metal migration between the two electrodes. When electrolyte is confined to scratches or narrow channels, the flow rate of the liquid is proportional to the voltage applied12.
Conductive filament formation exhibits a dependency on the interaction of temperature and relative humidity12. The effect of temperature and relative humidity on time-to-failure due to filament growth is combined as the percentage of moisture absorbed. The effect of moisture absorption depends on the temperature the laminate is subjected.
For a given relative humidity, the percentage moisture absorbed at temperature is higher than the percentage of moisture absorbed for the same relative humidity. The elevated temperature in the reflow process may cause the resin/fiber bonding to separate, thus increasing the chance of path formation.
The mismatch of properties including thermal coefficient expansion between resin and glass fibers can cause degradation of the bond between glass fibers and epoxy resin12.
The time-to-failure due to conductive filament formation in accelerated conditions appears to be higher for lead-free soldering12. The absorption of moisture is known to occur more rapidly at higher temperatures. A minimum threshold of humidity coupled with temperature creates a condition for the failure mechanism to occur. Post conformal coatings, applied after the populated circuit is soldered and cleaned, protects conductors, solder joints, and components during usage12.
Case Study #1Background: A company that manufacturers aqueous control systems moved away from eutectic tin-lead solder controller boards and into lead-free controller boards. The densely populated mixed technology circuit board was soldered with organic acid solder paste and wave flux. Following completed assembly, the board was cleaned in a DI water batch wash process.
Problem Statement: The electronics on the lead-free control circuit board experienced a number of field failures. The control device is located in a humid environment. In the presence of a humid environment and electrical bias, excessive ionic contaminants on the board can cause electrical failures. The boards in question were lead-free soldered but cleaned.
Failure Detection Analysis: The failed lead-free circuit boards were sent to a failure detection lab to determine root cause. Visual inspection of the circuit board found the presence of extensive flux residues and dendritic growth on fine pitch leaded devices.
Electrochemical Migration in the form of massive dendrites formed a bridge, which lead to spark events and charring the fine pitch component (Figure 4).
Data Analysis: Flux residues were not totally removed from fine pitch components. The boards were cleaned using hot DI-water in a batch cleaning process. Lead-free processed boards experience higher peak reflow temperatures.
As soldering temperatures rise, water soluble flux materials may undergo changes in their physical and chemical properties such as the evaporation of volatile fractions, and surface energy. Lead free flux residues are harder to clean. In this case, water only was not sufficient to clean the residue around the fine pitch leaded devices.
Corrective Action: Boards were evaluated for cleanliness effects. To improve the water soluble flux dissolution rate a 10-12% solution of an electronic engineered cleaning material was added to the DI-water wash process.
The aqueous cleaning material effectively removed the flux residues from the fine pitch components. Control modules were installed on field units. The field units will be monitored to determine correction action success or failure.
Case Study #2 Background: Lead Free cable harness is hand soldered with a no-clean core wire flux. The harnesses are used on high voltage devices.
Problem Statement: The surface tension of the no-clean flux residue wicks through the though-hole onto the gold contact points.
Field failures occurred from extensive dendrites and electrochemical migration on the contact fingers (Figure 5).
Failure Detection Analysis: The no-clean lead-free flux residue in the presence of humidity and electrical bias formed dendrites on the gold fingers. The lead-free flux residues left an ionic residue that conducted when powered in use conditions.
Data Analysis: To determine the integrity of the no-clean flux residue electrochemical failure analysis studies are in test.
Corrective Action: Failure analysis studies are in test to determine the effectiveness of the no-clean process. Removal of flux residues may be needed in this case.
The change to lead-free manufacturing urges tighter process control. Higher I/O counts, progressive miniaturization, more reliable performance, and other demands are being impeded by circuit board cleanliness effects. The relevance of clean lead-free assemblies must be considered to mitigate stray leakage and electrochemical migration reliability risks.
SAC lead-free alloys exhibit a poor wetting ability. More aggressive flux chemistries are needed to achieve wetting comparable with eutectic tin-lead alloys. For high reliability product suppliers no-clean flux technology introduces a difficult challenge. One critical problem is the lack of thermal stability, thus requiring more active flux compositions. This leaves a residue that no longer fits the no-clean classification.
Contamination can severally diminish the ability to resist shorting of leads and traces. Pre and post-assembly ionic and non-ionic contaminants are considered harmful residues to the electronics manufacturing process.
Localized contamination might not raise the contamination level of the overall assembly but may have a detrimental localized effect. This paper makes a strong argument from the literature to the importance of clean lead-free assemblies.
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10. Murai, H., Fukuda, T., Fischer, T. (2008). The evaluation of CAF property for narrow TH pitch PCB. IPC Printed Circuits EXPO. Las Vegas, NV.
11. Naisbitt, G. (2008, March). Cleanliness testing on the shop floor
12. Rudra, B., Pecht, M.J., & Jennings, D. (1996). Electrochemical migration in multichip modules. Circuit World. 22(1), 67.
13. Munson, T. (1998, Nov). Eliminating metal migration failures. Printed Circuit Fabrication. 21(11), 32.
14. Schweigart, H., & Wack, H. (2007, April). Humidity and pollution effects on Pb-Free assemblies. Circuits Assembly. 18(4), 34.
15. Seatman, K. & Nishimura, T. (2008). Properties that are important in lead-free solders. IPC Printed Circuits EXPO. Las Vegas, NV.
16. Tiggelen-Aarden, I.V. & Westerlaken, E. (2008). Performing flux-technology for Pb-free SN100C solders. IPC Printed Circuits EXPO. Las Vegas, NV.
Dr. Mike Bixenman is the Chief Technology Officer of Kyzen Corporation.
Please email observations, comments, and questions to mikeb@kyzen.com
