Abstract
The distances between conductors, and the under clearance gaps from the board to the bottom of the components on printed circuit boards, are smaller due to miniaturization. Smaller spacing increases the probability that flux residues or surface contamination will be suffi-cient to bridge all or most of the under clearance gap between conductors. Flux bridging conductors opens the pathway to form a conductive cell between two points on the board assembly. As a result, higher density board designs increase reliability risks, which are commonly mitigated by cleaning all flux residues and ionic contamination on the surface and under components on the assem-bly. One of the problems is that the low spacing under components requires in-creased wash time and pressure for the cleaning agent to penetrate and remove all flux residues under component gaps. The purpose of this research is to study cleaning process factors and levels as a function of the gap height.
Electronic device innovations result from smaller spacing between conductors thus requiring more active circuitry within highly dense devices. Contamination and its effects are more problematic as electronic circuit spacing between conductor’s decreases.
The long-term effects of no-clean pro- cessing on reliability of printed circuit boards continues to be a source of concern, especially when coupled with reductions in lead pitch and conductor spacing.1 Further reliability risks are introduced with the transition to lead-free solder technology, since these alloys typically include silver and are processed at higher temperatures than eutectic tin-lead solder.1
Proper cleanliness levels on assemblies built with highly dense component place- ments, component configurations (low clearance with a lot of flux underneath) and miniaturized components have become more difficult to achieve. Assemblers must better understand the long term effects of these assembly residues.2 The smaller spac- ing between conductors yields a larger elec- tric field, which in conjunction with insuf- ficient cleaning, can lead to device failures. Current industry specifications for cleanli- ness may be inadequate for the next gen- eration of electronic assemblies.2
Background
The distances between conductors, and the underneath clearance gaps from the board to the bottom of the components on printed circuit boards, are smaller due to miniaturization. Smaller spacing increases the probability that flux residues or surface contamination will be sufficient to bridge all or most of the under clearance gap between conductors. Flux bridging conduc- tors opens the pathway to form a conduc- tive cell between two points on the board assembly. As a result, higher density board designs increase reliability risks, which are commonly mitigated by cleaning all flux residues and ionic contamination on the surface and under components on the assembly.
Cleaning flux residues from under component gaps has become extremely chal- lenging due to the changing nature of the flux residue, under component clearance from the board to the bottom of the component, time required for the cleaning agent to penetrate the gap, the cleaning agents ability to solvate and break the flux dam needed to create flow channels, and the mechanical energy needed to deliver the cleaning agent to the flux residue. Flux residues that form a hard shell require longer wash times to dissolve in the cleaning agent, thus requiring increased time to clean these residues under the component gaps.
Five cleaning forces
Five forces that directly influence the cleaning properties of flux residue are the solder flux, heat exposure, component gap, cleaning agent, and cleaning equipment (Figure 1). Variations in any of these factors can and does influence the cleaning rate.
Figure 1: Five Cleaning Forces
Soldering flux
Solder alloys rapidly oxidize upon exposure to air, moisture, and heat.3 Oxida-tion is caused by exposure to oxygen in air, which results in a non-conductive and non-solderable metallic surface. Solder flux is a chemical cleaner that removes oxidation form metal surfaces, facilitates wetting, and improves metallurgical bonding. When flux is heated, low boiling constituents within the flux evaporate, flux activators remove surface oxidation, and oxygen barriers (rosin/resins) protect the alloys from reoxidation during the solder process. During the soldering process, heating and cooling ramp rates must be compatible with the assembly and components. The time of exposure to high temperatures must be defined and maintained.4
The soldering process can be affected by the mass of the associated component, proximity and mass of neighboring components, the size of the pads, and the amount of heat that travels through the tracks and boards.4 These factors increase demands on the flux, which has a significant influence on quality and low defect soldering rates. This task becomes more difficult with highly dense miniaturized designs and lead-free soldering. To address these complexities, the flux must be stable to high temperatures; resist charring, oxidation and burning; and provide a resistant oxygen barrier.5 These properties change the solubility and cleaning properties of flux residue post soldering.
Flux and cleaning agent advances of the past 20-years have kept pace with compo- nent and board assembly technology ad- vances. Rosin, low-solids, no-clean, and water soluble flux technologies designed for eutectic tin-lead were readily cleanable even after multiple soldering processes. The same cannot always be stated for lead-free soldering. Miniaturization and lead- free soldering require more active and stable flux compositions that remove oxi- dization with less flux, wet higher surface tension alloys, and protect the underlying metal from oxidation during the soldering process. The cleaning properties of lead- free flux compositions, including water soluble, have changed. The residues are harder and require more active cleaning agents and mechanical to remove the flux residues.
Heat exposure
The reflow process heats the circuit board plus components held by solder paste through successively higher temperatures. The solder profile progressively starts by evaporating flux volatiles, initiates flux activation, raises the components to be joined to a temperature which is sufficiently consistent for the solder to flow evenly onto all surfaces, and reflows the solder paste over board finishes to facilitate solder con- nections. Temperature excursions and the time exposed to liquidus solder temperatures influence cleaning properties.
Excessive exposure to the soak and liquidus stages can oxide (char), crosslink (polymerize) and harden flux residues. Figure 2 illustrates a lead-free solder pro- file using a soak process near liquidus. A long soak profile ensures that the solder paste is fully dried before hitting reflow temperatures. In this example, the heat generated from the flux activation zone cross-linked the flux residue. To clean this residue, an aggressive cleaning condition is needed to remove cross-linked residues. In some cases, the cleaning process win- dow is very or quite narrow, resulting in highly inconsistent cleaning.
Figure 2: Soak Reflow Profile
Optimal soldering processes are hot enough to enable the solder to wet the board and components yet cool enough not to damage the items being soldered and a controlled cool down to ensure solder joints are sound.4 Defining the exposure to high temperature requires heating and cooling ramp rates that are compat- ible with the solder paste, components, board finishes, and cleaning process. Fig- ure 3 illustrates a ramp to spike lead-free solder profile using the same solder paste illustrated in Figure 2. Reduce heat exposure rendered a cleanable flux residue.
Figure 3 :Ramp to Spike Reflow Profile
Bake out cycles post cleaning expose flux residues to excessive heat over long periods of time, which can char, polymerize, oxidize and harden flux residues. In some cases the residues are not cleanable using common production cleaning processes. The image illustrated in Figure 4 was from a board baked over the weekend at tem- peratures exceeding 120°C. The charred flux residue was not cleanable using the current production cleaning process.
Figure 4: Post Bake Before Cleaning
Changes to normal process procedures can also change the nature of the flux residue. This change can render a residue that when processed under normal conditions is cleanable, but when exposed to long bake out cycles at elevated temperatures renders a residue that is not cleanable.
Component gap
Component miniaturization decreases the spacing between conductors. During solder reflow, flux under fills the bottom side of the component (Figure 5). The distance from the board surface to the bottom side of leadless components is consistently less than 2 mils. For cleaning to occur, the cleaning agent must first wet the residue. To sufficiently wet the residue, the cleaning process must break through the flux dam to create a flow channel.
Figure 5: QFN Component with less
than 2 mils Gap
Cleaning under low gap components is increasingly more difficult due to the high- er molecular weight non-polar covalent resins being formulated into lead-free flux compositions. With clearance gaps under components of less than 2 mils, and for small chip caps, gaps less than 1 mil cre- ates a highly difficult cleaning challenge (Figure 6).
Figure 6: Chip Gap Height of .6-.8 mils
The higher molecular weight resins formulated in no-clean solder pastes require in- creased solubility and mechanical forces to enable the flux residue to dissolve into the cleaning agent. This force of attraction de- pends upon the nature of the cleaning agent and the nature of the flux residue. Penetrating low clearance gaps requires a cleaning agent that matches up with the flux residue, along with impinging forces that can deliver the cleaning agent to the residue.
On soft residues, such as the water soluble soils, flux residues are much easier to clean under low gap components. Pen- etrating the residue and creating a flow channel occurs rapidly. Conversely, for hard no-clean flux residues, the time to clean all residues under the component gap can be five to ten times greater than the time required for a soft residue.
Cleaning agent
The critical differentiator for removing higher molecular weight flux residues is the cleaning agent. The ideal cleaning agent is formulated with the greenest environmental properties within performance limitations; rapidly dissolves polar protic, dipolar aprotic and non-polar soils; and is easily rinsed leaving an ionic cleaned assembly. Since flux residues are a composi- tion of rosin, resins, activators, rheological additives and reacted ionic salt forms, the cleaning agent requires a composition of materials that remove polar protic soils, dipolar aprotic soils, and non-polar resins.
One of the lead-free flux residue differences is the increase in non-polar resins. To clean flux residues with high molecular resin structures, a greater level of solvency is required. Engineered cleaning agents provide a viable approach toward meet- ing the requirements for cleaning lead-free flux residues.
Cleaning agent can be engineered with materials that target ionic, polar covalent and non-polar covalent materials found in the flux residue. In order to classify the materials that make up the cleaning agent, the concept of “like dissolves like” guides the formulators understanding of the forc- es needed to enable the flux residue to dissolve in the cleaning agent.
Research into many of the lead-free flux residue properties indicate a much broader range of resins used than what was previously applied to tin-lead solder paste formulations. With different solder paste suppliers using a wide range of resin/rosin materials, the attraction of the cleaning agent for the soils will be different. With this increased level of complexity, there will be a greater requirement for matching the cleaning agent to the soil.
The challenge in designing cleaning agents for lead-free soils is that residue properties are very different amongst solder material companies. Depending on the properties of the residue, some clean- ing agent designs must drive with stronger forces than needed for soft residues. Up- stream factors can change cleaning properties. As a result, there is “rarely” one best cleaning agent since the nature of today’s cleaning process is far more inconsistent than years past.
Cleaning equipment
With lower component heights, the cleaning equipment must deliver the cleaning agent to the flux residue. Visible flux residues before cleaning on leaded devices (Figures 7 & 8), even on fine pitch compo- nents, are typically not an issue to clean. The issue is cleaning flux residues under component gaps on leadless components.
Figure 7: Visible Flux Residue before Figure 8 : No Visible Flux Residue after
Cleaning Cleaning
To remove flux residues under component gaps on leadless components, cleaning equipment designs that deliver the cleaning agent to the source of the flux residue using spray in air are commonly used. Typical spray in air cleaning machines are equipped with spray nozzles that hit the board surface and then deflect to move the cleaning agent to the residue. To improve deflective energy, tighter spray nozzles de- liver cleaning agents at higher pressures and fluid flows, which provide higher impact forces with less pressure drop. Once the spray hits the board surface, the deflec- tive energy forces the cleaning agent under the component gap.
The problem with this approach is that the position of the board to the spray pat- terns is commonly not consistent over the board’s surface. As such, some com- ponents are cleaned while others com- ponents on the same board still have flux residue remaining under the leadless com- ponent. To improve this condition, some cleaning machine companies are angling the spray nozzles so they hit the board at the leading edge of the component (Figure 9). Spray patterns that hit at the edge of the component are not as susceptible to energy losses and provide improved consistency for removing residues under leadless components.
Figure 9: Angled Spray Nozzles hit at
Edge of Component
To illustrate, consider a 10×10 mm µBGA with 0.4mm Pitch (Figure 10).
Figure 10: Area Array µBGA
Figure 11 illustrates a µBGA with 0.40 mm pitch cleaned with a spray pattern using deflective spray in air energy. Flux residue remained under the device.
Figure 11: Flux Residues under µBGA Post Cleaning
Figure 12 illustrates the same µBGA cleaned with angled patterns using spray in air energy. No flux residue remained under the device.
Figure 12: No Flux Residues under µBGA Post Cleaning
With gap heights less than 2 mils and tightly pitch interconnects, cleaning ener- gy must be optimized to achieve cleaning consistency.
summary of 5-cleaning forces
Variability in the five cleaning forces impacts cleaning consistency. Understanding the five cleaning forces and how they apply to a particular cleaning process differentiates a process that meets the cleaning objective versus one that fails to meet the process objective.
Each of the 5-forces are multi-faceted and must be studied to understand their influence on cleaning the flux residue from under leadless components and then integrated to achieve process consistency. Failure to consider each of the 5-forces when designing the cleaning process often leads to a poor cleaning process.
Methodology
As components decrease in size and functionality, the clearance gap height under components diminishes. Gap heights of less than 2 mils are common for many leadless components. Lower gap heights and tighter pitch between conductors result in flux residues underfilling the component’s bottom side. Stencil manufacturers can improve this condition by designing step stencils with different aperture shapes that allow the component to be reflowed with higher gap clearances.
The purpose of this research is to test visual cleaning under components populated on the IPC-B52 Test Vehicle (Figure 13). The stencil design and placement of components left clearance gaps (standoff heights) of 2 mils and greater for most leadless components. Two no-clean solder pastes were tested, one eutectic tin-lead solder paste and one SAC lead-free sol- der paste. The boards were processed in a batch spray-in-air cleaning machine using an engineered aqueous cleaning agent.
Figure 13: B-52 Test Vehicle
Research Hypothesis: The removal of flux residues under low gap components is improved when gap heights from the substrate to the bottom of the component exceeds 2 mils.The gap heights on some of compo- nents placed on the B-52 test vehicle were measured.
Data findings
All components were removed from control (not cleaned) and cleaned eutectic tin-lead no-clean and SAC lead-free no-clean boards.
Inferences from data findings
The successful cleaning under lead-less components was a function of the 5-cleaning forces cohesively aligned. The solder flux was not overheated, which led to an easily cleanable residue. The cleaning agent matched up to the flux residue, which allowed the soils to rapidly dissolve into the cleaning media. The high compo- nent gaps exposed more surface area to the cleaning agent improving the disso lution rate. The cleaning equipment fluid flow and impingement energy moved the cleaning agent underneath the component and eventually created a flow channel that removed all residues.
Other cleaning design factors also contributed to the level of success. For this experiment, a wash time of 20 minutes was used. From past research, wash time con- tinuously comes up as a critical factor for cleaning underneath components. One of the benefits of batch systems is that longer wash times can be programmed in with- out creating a bottle neck.
The one component that showed a low level of flux residue underneath the device was the QFN.The problem with the QFN is the ground plain in the center of the component. The ground plain inhibits a flow channel. To cleaning the device, spray impingement must hit the four sides of the component. One of the limitations of the batch system used for this test is that there were no side spray manifolds. Without consistency of spray on all four sides the probability of some residue is high.
concluding remarks
The age of miniaturization improves response time with shorter distances between conductors. The under clearance gaps from the board substrate to the bottom of the components are smaller due to miniaturization. Smaller spacing increases the probability that flux residues will be sufficient to bridge all or most of the under clearance gap between conductors. Flux residues bridging conductor’s opens the pathway to form a conductive cell, which increase reliability risks.
To clean under components, the clean-ing agent must wet, dissolve the flux residue, penetrate under the gap, and create a flow channel. The data from this research finds that higher standoff heights improve cleaning performance. Working upstream with stencil manufactures and component placement equipment manufactures to study stencil designs, paste thickness, aperture shapes, and depth from which the components are placed into the solder paste may improve gap heights. Increasing the gap heights to a minimum of 2 mils reduces cleaning variability and improves the probability that all flux residues will be removed under components.
Components with gap heights less than 2 mils can be cleaned at elevated process conditions. Typically, longer times in the wash are higher levels of impingement energy are required.
REFERENCES
1.Azarian, M.H. (2010, Feb.). Electrochemical Migration in Printed Circuit Board Assemblies. Telecom and Webex. CALCE EPSC, University of Maryland. College Park, MD, 20742.
2.Bumiller, El, Pecht, M., Hillman, C. (2010). Electrochemical Migration on HASL Plated FR-4 Printed Circuit Boards. CALCE Electronic Products and Systems Center. College Park, MD, USA.
3. Pippen, D.L. A Primer on Hand Soldering Electrical Connections. New Mexico State University.
4. Tarr, M. (n.d.) Wave Soldering. Creative Commons Attribution – Non Commercial – ShareAlike 2.0.
5. Lee, N.C. (2009). Lead-Free Technology and Influence on Cleaning. SMTAI, San Diego, CA.