Belt speed, zone temperatures and static pressure all impact peak temperature.
The change from eutectic to Pb-free solder has required new recipes for reflow ovens. Solder paste manufacturers have developed profile parameters, but it remains the responsibility of the assembly engineer to find the correct control settings with a limited number of reflow oven adjustments. Additional complications exist because the liquidus temperatures of the new solders require peak temperatures close to the point at which components are damaged. Thus, the importance of accurate recipes and precision oven control is amplified.
Conventional reflow ovens have two adjustments for profile development. One is the zone set points and the other is belt speed. Some oven manufacturers have added high, medium and low fan speeds as an additional adjustment, and one manufacturer has a closed-loop pressure control.
Given the three possible adjustments, a study was undertaken to determine the effects varied belt speeds, static pressures and zone temperatures have on the peak temperature, time above liquidus (TAL) and temperature uniformity of a 100 and 230 g populated surface mount board.
Experimental
A Pyramax 98 N reflow oven with edge rails, fine mesh belt, and closed loop pressure control was used for this experiment. A recipe that produced a Pb-free ramp-to-peak profile with a belt speed of 28 IPM and static pressure of 1.0 IWC was chosen as a baseline (Table 1).
The plan was to individually vary each parameter (high and low) and record its effect on the TAL, peak temperature and uniformity of each board. An additional run was performed with all variables at the high and low settings to see the combined effect.
High and low ranges were established for each of the variables, as outlined in Table 2. Data were gathered with a SlimKIC II profiler.
Results
The 100 and 230 g boards were run at the baseline operating parameters of 28 IPM, 1.0 IWC and zone 7 set points of 250°C. Figures 1 and 2 show the resultant profiles for the 100 and 230 g boards, respectively. Table 3 shows the peak temperature and TAL data for the boards. There was a difference of about 5°C at the peak and 8 sec. in the TAL between the two boards because of weight and board design.
Belt speed. Belt speed was varied from 24 to 32 IPM with the static pressure and zone set points at the baseline settings. Table 4 shows the peak temperature, TAL and uniformity data for each board. The increased belt speed lowered the peak temperature and TAL, and slightly decreased the temperature uniformity at the peak.
Zone temperature. The oven was reset to the baseline parameters, and the temperatures in zones 6 and 7 were increased and decreased by 10°C. Peak temperature and TAL increased with the higher zone temperature settings and the uniformity decreased (Table 5).
Static pressure. The oven was reset to the baseline parameters and the pressure varied from 0.7 to 1.3 IWC. The increased static pressure increased the peak temperature by about 5°C and TAL by about 10 sec. (Table 6). The uniformity at peak was significantly better with the higher static pressure.
High and low interactions. Next, the combination of all the high temperature parameters (low belt speed, high zone set points, and high static pressure) and low temperature parameters (high belt speed, low zone set points, and low static pressure) was used to determine the interactions on each board. There were significant changes in all the profile attributes, with about 30°C differences in peak temperature and close to 50 sec. in TAL (Table 7). Uniformity was considerably better with the high oven parameters. In the case of the heavy board, the peak temperature did not reach the liquidus when all the settings were set low.
Today’s high performance reflow ovens have three adjustments that permit recipes for solder reflow profiles. A recipe that works for one board won’t necessarily work for another board if the weight or design is significantly different.
Of the three oven adjustments, the zone set points have the biggest effect on the peak temperature and TAL. Changing the belt speed also affects the peak temperature and TAL, to a lesser degree. But the static pressure not only affects the peak temperature and TAL, it has the biggest impact on uniformity at peak temperature.
Pb-free solder’s more stringent process requirements make it important that all three adjustments – zone temperature set points, belt speed and static pressure – be used when developing recipes.
Fred Dimock is senior process engineer at BTU International (btu.com); fdimock@btu.com.
A thinly plated hole could be susceptible to PTH lead damage.
The latest addition to the database is delamination on the surface of a board assembly. A microsection (Figure 1) shows a plated through-hole with a through-hole lead after soldering using a PbSn process. Prior to sectioning, the board had shown evidence of minor delamination and measling around the pad area on the board subsurface. The board was being produced in medium volume for a consumer product.
Mechanical strain or damage may occur during pin insertion. The soldering temperature or the time to solder may have been excessive, causing board expansion. The image in Figure 1 is not very clear, but based on the examination and comparison of the relative dimensions, the plating in the hole is thin, probably less than 20 µm. There is evidence of delamination/separation of the glass bundles at the hole-copper interface.
Close examination of the microsection and the rest of the board will be required for root cause analysis. There are a number of possible causes, and along with the board examination, all the process details would be reviewed, or even the assembly/soldering operation audited.
These are typical defects shown in the National Physical Laboratory’s interactive assembly and soldering defects database. The database (http://defectsdatabase.npl.co.uk), available to all Circuits Assembly readers, allows engineers to search and view countless defects and solutions, or to submit defects online.
Dr. Davide Di Maio is with the National Physical Laboratory Industry and Innovation division (npl.co.uk); defectsdatabase@npl.co.uk.
A new system weighs the impact of proposed changes.
A basic premise in Lean manufacturing principles is the elimination of non-value added activity by minimizing variation. However, standardizing systems while accommodating the needs of 30 or more customers can be a significant challenge for EMS providers. Customer satisfaction measurement is one area where standardization can improve efficiency. Yet, the more standardized the format, the less meaningful it may be at the customer level.
Like many contract manufacturers, Epic ran a dual customer satisfaction system. Epic participated in its customers’ ratings systems and conducted monthly and annual web-based customer satisfaction surveys. However, only 8% of customers were sending formal monthly scorecards defining their expectations. Of the remaining customers, only 20% were filling out monthly surveys, and the surveys tended to generate subjective responses that did not necessarily tie to specific improvement activities or clearly defined goals.
In the fourth quarter 2007, the company’s management team decided to put a new system in place that would:
- Work seamlessly across multiple facilities.
- Better align with each customers’ measurement criteria (expectations of Epic performance).
- Deliver quantitative feedback, even on softer areas of evaluation such as quality of team interaction.
- Ensure that both Epic’s customer focus teams (CFT) and operational management had continuous visibility into customer issues and status of corrective actions.
- Integrate with long-term objectives for account growth.
The new system was fully implemented in 2008, then re-evaluated and fine-tuned last March.
System overview. We manage projects using a Customer Focus Team model. Each CFT includes a program manager, account manager, quality engineer, product engineer, test engineer, material analyst and inside salesperson.
Early in our operational strategy formulation, management developed a methodology for measuring and sharing performance information, known as the Plant Operational Review (POR) system. The original version monitored approximately 60 metrics company-wide down to the floor level. These metrics were formally reviewed on a daily/weekly basis by project personnel, monthly by plant managers, and quarterly by senior management. Over time, the system has evolved to include the original metrics list, external benchmarks and longer-term performance trends. The POR process starts with a summary of overall company financial performance metrics, then focuses on specific productivity and operational performance in human resources, quality, manufacturing, engineering, sales, purchasing and finance. The functional managers responsible for performance to measured metrics are also responsible for defining the external benchmarks relevant to their areas.
During the gap analysis of the former Customer Survey System, opportunities for improvement were identified that could tie into the management review cycle (POR) to close our internal loop. One such opportunity was to create a working tool for use by the CFT and customer that would define expectations based on the monthly survey. The redesigned customer satisfaction measurement tool was named the Customer Expectation Worksheet. A goal for the new system was that it link to POR, showing both customer issues and the status of corrective actions related to those issues. Another goal was to link the customer satisfaction survey closely with other program management tools. One key tool that was developed was the CFT Tracker.
The CFT Tracker is a living diary of each customer. It is an Excel workbook resident on the company’s intranet that includes tabs for core customer team contact list, product/part number lists, NPI planning, meeting agenda, CFT open action items, continuous improvement team (CIT) tracker, CFT Paynter chart, customer PPM tracking, scrap analysis, closed CFT action list and the Customer Expectation Worksheet. In short, the CFT provides the entire account history and current status information at the fingertips of anyone within the organization.
Because the CFT Tracker stores trends information related to quality and continuous improvement initiatives, it enables real-time analysis of customer issues identified through the Customer Expectation Survey and makes it easy for the CFT to respond with specific data related to issues identified by the customer.
For example, if our team has made DfM recommendations that are currently affecting manufacturing, on-time delivery, quality, etc., then this will be tracked in the CFT Tracker through the Paynter charts. If the customer has opted not to adopt the recommendations, but indicates in its monthly survey that defects exceed predefined limits, the CFT can look at the CIT tracker, PPM data and CFT Paynter to determine what percentage of defects relate to the unadopted DfM recommendation. With the Paynter chart, the team can show a weighted analysis of the impact of adopting the proposed changes. Similarly, if the defects relate to an out-of-control process, we would have the data to drive internal improvements. The result: a hyper-focused corrective action tool. Training was conducted at all Epic facilities. In Mexico, training was conducted in Spanish to ensure full understanding among all CFT members.
The survey process. The Customer Expectation Worksheet was designed as a relatively simple tool. We have defined a series of ratings tied to quality, delivery performance and partnership. Each of the three sections has three-to-four defined performance indicators. Within each performance indicator, respondents rate on a 1 to 5 scale:
- 5 – Frequently exceeds requirements (green color code).
- 4 – Occasionally exceeds requirements (green).
- 3 – Meets requirements (yellow).
- 2 – Occasionally does not meet requirements (red).
- 1 – Frequently does not meet requirements (red).
The CFTs work with their customer to establish a customer-specific metric for each of these defined performance indicators. Customers then rate on the 1 to 5 scale against their predefined performance metrics. Although a rating of 3 indicates requirements are met, it is coded yellow and reported as an opportunity for improvement. Ratings of 1 or 2 are coded red and generate a corrective action requirement, which is tracked at both the CFT and POR level.
Each CFT provides a list of key contacts that Epic interacts with on a day-to-day basis. The survey is sent as a web-based choice board form to customer contacts. Requests are rotated among the total list of core contacts so that each contact only gets a request a couple times a year. If there is no response to the initial survey request, up to two reminders are sent out. If the customer contact still doesn’t return a survey, the CFT will touch base to determine the reason why.
Also, a more detailed annual Customer Loyalty Survey conducted via email measures:
- Overall performance satisfaction compared to satisfaction with other EMS providers.
- Perception of management and key support competencies.
- Relationship with project team.
- Responsiveness to problems.
- Perception of price competitiveness.
- Plans for future business allocation.
This annual survey is sent to multiple contacts at each customer and includes areas for detailed comments and suggestions for improvement. Survey data are reviewed at the plant and corporate level.
Results and lessons learned. When the new survey was deployed in 2008, it consistently generated a 45 to 50% response rate, compared to the prior 20% response rate. In 2009, that has dropped to about 30%, but that percentage typically includes 100% of our largest customers. In determining survey improvements for 2009, one issue has stood out. Customer project teams prefer to do the survey as a group. When individuals are contacted for a survey, they often solicit feedback from other members of the core customer team. If they do not get feedback, they often do not return the form. As a result, a 2009 change to the survey method will be to offer the customer the opportunity to complete surveys from each core customer group, rather than to attempt to rotate between those team members.
An additional indicator of the robustness of the process is that we won all five individual service category awards in our revenue size class in Circuits Assembly’s 2009 Service Excellence Awards for EMS providers.
Tony Bellitto is quality manager-US Operations at Epic Technologies (Epictech.com); tony.bellitto@Epictech.com.
When the solder doesn’t stick, first check the pad.
Dewetting is a condition that results when molten solder coats a surface and then recedes, leaving irregularly shaped mound(s) of solder separated by areas that are recovered with a thin film of solder and with the basis metal unexposed.
Non-wetting is a condition in which there is partial adherence of molten solder to a surface it has contacted, and the basis metal remains exposed.
While we usually list in this space the primary process setup areas to check, dewetting and non-wetting typically are board-related due to pad surface contamination.
Other things to look for in the process include:
- Solder temperature too low.
- Preheat too high or low.
- Excess or insufficient flux blow-off.
- Solder wave height low.
- Flux not making contact.
- Flux contamination.
- Board pallet too hot.
- Flux applied unevenly.
- Flux SP GR too low.
- Conveyor speed too fast or slow.
- Board not seated properly.
- Flux SP GR too high.
- Solder contamination.
- Other things to look for with the assembly include:
- Board or component contamination.
- Improper board handling.
- Other things to look for with the board design include:
- Oxidation.
- Contamination.
Paul Lotosky is global director - customer technical support at Cookson Electronics (cooksonelectronics.com) plotosky@cooksonelectronics.com.
Don’t maintain the status quo if potential to reduce manufacturing failures exists.
An inquiry led to an investigation on the possible causes of printed wiring board failures, which were becoming increasingly prevalent after SMT manufacturing. Failures were detected by electrical testing, but the location and specific devices causing the failures were undetermined. The failures were suspected to be predominantly in the BGAs located on specific sites on this 16-layer construction. Failure data provided included high resistance shorts occurring in those specified areas. The surface finish was a eutectic HASL, and the solder paste was a water-soluble SnPb.
The diagnostic approach agreed upon included an examination of both the quality of the manufacturing process and the materials used for assembly, as reviewed below.
SMT process. The first order of diagnostics, a manufacturing audit to assess the SMT process, revealed:
Solder paste was properly stored and permitted to reach ambient conditions prior to use. The solder mesh was appropriate for the type of assembly, and the paste was not expired.
Stencils used for paste application were properly proportioned with an aperture size appropriate to the BGA device pitch.
The stencil printing operation revealed no flaws, and paste was applied in a smooth, consistent manner with uniform height and width.
The paste reflowed uniformly and covered leads per IPC-A-610D class 3 specifications. No apparent skips or dewetting were noticed on resistors, capacitors or leaded chip devices.
The reflow oven has four zones and short length, but no issues around reflow quality were seen as a result. The recommendation was made to increase to a seven-zone oven to increase profiling flexibility for various designs.
Reflow profile. The reflow profile was in the specified range for the solder paste of choice (Alpha WS-809). Thermocouple placement was in appropriate locations, and the temperature uniformity among each location reflected an even distribution of heat. The time above liquidus (TAL) varied by only 6 sec. between the highest and lowest time, while the peak temperature varied by 13˚ among locations. There was no evidence of hot or cold spots.
The assembly TAL averaged 90 sec., well within the vendor-recommended TAL range of 40-120 sec. As reflow was clearly achieved, and there was no evidence of cold solder joints, a recommended step was to decrease the TAL from 90 to 60 sec., which would improve wetting without a risk of incomplete solder reflow. However, the present reflow profile is consistent with the recommended parameters prescribed by the solder paste vendor.
Bare board inspection. A visual assessment of the bare boards showed evidence of solder mask overlap into the pad areas, exposed copper under the HASL surface finish, and non-uniform HASL finish in certain cases.
XRF inspection. BGA devices and bare board were analyzed using x-ray fluorescence to determine the alloy composition of the metal (Tables 1 and 2). This is a common method used when the composition of the component alloy or surface finish is in question.
XRF summary. Components indicate a eutectic or near eutectic SnPb composition. Analysis of the HASL finish indicated a non-uniform thickness, and at certain locations, the finish was thin enough to permit the underlying copper to overwhelm the alloy analysis.
X-ray analysis. X-ray analysis was performed on a populated board at the various BGA locations suspected as problematic. No evidence of shorting opens or misalignment was prevalent.
Optical endoscope. The populated assembly was analyzed through an endoscope to see any evidence of incomplete collapse or other observable phenomena such as dewetting or head-in-pillow effect. No unusual occurrences such as excessive solder or flux residue were visible. However, the graininess of the solder balls may indicate the start of an oxidizing surface. This can be due to excessive time in liquidus state. The solder balls seem to be well collapsed and formed.
Wetting balance. The wetting balance test showed significant issues with the wettability of the HASL board at various locations (Figure 1). J-STD-003 criteria suggest time to buoyancy corrected zero, T0 (where thewetting force goes positive), should be less than 1-2 sec. In every sample tested, T0 was greater than 5 sec.
Conclusions and Recommendations
The HASL surface finish is non-uniform, with copper exposure and very thin layers of HASL in some areas.
The wettability is well below the recommended wetting criteria, as specified by IPC specifications, and can be a primary concern during assembly. Residual oxides on the HASL surface may prevent proper wetting, or form weak intermetallic interfaces when reflowed. The type and amount of oxide residue can be ascertained through Sequential Electrochemical Reduction Analysis (SERA).
The x-ray images showed no evidence of poor alignments, opens or shorts.
Optical endoscope images showed no anomalies and good BGA ball collapse.
ENIG should be considered as a board surface finish instead of HASL, if available. It is the most common finish for BGAs, and would eliminate the non-uniform surface finish.
Application of a more uniform HASL surface would ensure complete coverage of the underlying pad and alleviate exposure of the underlying copper, while reducing the risk of oxidation.
The HASL bath should be analyzed for contamination from excessive metals and other residuals from the soldering process.
Reducing the liquidus time by 30 sec. may help alleviate excessive formation of oxides on the solder.
A seven-zone oven would permit more flexibility in shaping the reflow profile around a varied array of designs and constructions.
Lessons Learned
Results of the analysis indicated that, although certain process and manufacturing improvements can be made to reduce both systematic and randomized failures, examination of the raw materials (for example, the PCB) can prevent a significant amount of manufacturing defects.
In this particular case (which is not atypical), a thorough investigation provided the engineer enough data to support the claim that the cause of electrical failure was due to noncompliancy in the surface finish that caused dewets. Establishing documented proof of failure can exonerate the manufacturing process, and prevent excessive and unnecessary expenses.
There is a natural inclination to avoid impugning an established process that has, for the most part, produced a successful product. However, the temptation to maintain the status quo should be avoided if the potential to reduce manufacturing failures exists. Most issues arise when an interaction between non-centered processes result in a multiplicative effect. In this case, the effect of poor distribution of the HASL surface, along with a marginalized reflow process, combined to produce failures that may not have occurred if either of the conditions for HASL or reflow was optimized.
The ACI Technologies Inc. (aciusa.org) is a scientific research corporation dedicated to the advancement of electronics manufacturing processes and materials for the Department of Defense and industry. This column appears monthly.
Harmonizing all the parameters is no simple feat.
What are the best settings for a wave soldering process? The answer is not as simple as one might think. Much depends on the flux type and end-product. However, adherence to some basic rules will ensure a robust process. A good wave process depends on establishing correct machine and product parameters. Fluxing, preheating, conveyor speed, solder temperature, dwell time, wave height, wave type, nitrogen and exhaust are machine parameters, while board complexity, component types, flux type and pallet use are product parameters. All these parameters interact and therefore should be optimized to work in harmony.
A wave soldering process breaks down into the following categories:
Fluxing. The correct amount of flux to be applied per board is based on the flux supplier’s specifications. Excessive flux may interfere with the product’s electrical reliability, and a moderate amount of flux may not provide sufficient tail activity to reduce bridging and to obtain good through-hole penetration when the board leaves the wave. It is extremely important to optimize fluxer settings, which are related to conveyor speed. Visual testing should be used to ensure proper overlap and penetration of flux. With alcohol-based flux, thermal fax paper can be used on the bottom of the assembly and processed through the fluxer only. A visual footprint then can be seen and areas missed by the flux pattern identified. The same is true if the paper is applied to the top of an unassembled board. The paper must be fixed to the assembly to avoid movement during flux application. For water-based flux, pH paper can be used. Also commercial test fixtures can be used for flux test application. The appropriate flux type (i.e., alcohol- or water-based fluxes) depends on the application, board surface finish, solder resist, board complexity and other issues.
Preheating. Board preheating is necessary to evaporate flux solvents and to prepare the board and flux for soldering. Preheating also is used to reduce thermal shock of components and to promote better through-hole penetration, especially for multilayer boards. The flux supplier specifies preheating parameters. For the case of alcohol (i.e., isopropyl) fluxes, the board topside temperature should be above 82°C, and for water fluxes above 100°C. Complete evaporation of the solvent is important to reduce soldering defects such as openings, voiding, and solder balling. Depending on the type of flux and board thickness, higher preheat temperatures may be required to activate it.
When surface mount components (i.e. chip components) are glued to the bottom side of the board, it is important to ensure that the ΔT between the temperature of the components and solder temperature is between 100° and 110°C. It is important to follow the supplier’s specifications for wave soldering surface mounted components.
Conveyor speed. A typical conveyor speed setting will be in the range of 1 to 1.5 m/min. The speed setting depends on board complexity. Single-sided boards often can be soldered at high speed because they often have a low thermal demand and no plated-through barrel, and thus do not require topside fillets. A multilayer board may have a high thermal demand and 1 m/min could be too fast. To optimize the conveyor speed, it is also important to consider the board layout at the solder side, which can be a decisive factor to prevent solder bridging.
Soldering temperature. The solderpot temperature setting depends on the type of solder, but also may be related to the product to be soldered (i.e., board complexity, pallet use, or exposed bottom-side surface mount components). In general, low temperature settings are recommended to avoid board warpage and component damage. Lower temperatures create less dross, and extend the lifetime of the flux so that it has better tail activity. During soldering, the topside board temperature must be below the melting point of the surface mount component joints to avoid double reflow. For SnPb solders, 245° to 250°C is a common setting. For SAC alloys, 260° to 265°C is the recommended setting. It is important to keep the solder bath volume constant to maintain soldering temperatures.
Dwell time. A board must touch the wave for a sufficient time to make a good solder joint. The real contact of a joint depends on the protruding length of the leads and the board layout. The typical contact time for Pb-free applications is between 3 and 6 sec. The dwell time also may affect board warping. To avoid excessive warping, board supports or pallets can be used.
Wave height. The wave height should be kept low to minimize dross formation. In general, lead clearance of 6 to 8 mm relative to the bottom of the assembly to the wave formers in the solder pot is preferred. Lower settings may move components during soldering, as the leads may touch the nozzle rim. A wave height setting should be constant within a few tenths of a millimeter. For this reason, the solder level of the solderpot should be monitored and corrected automatically.
Wave type. There are three types of waves: chip, main and smart wave. Depending on the type of assembly and flux, it may be best to use one wave former. Chip wave is a turbulent wave and is used as first wave to enable wetting of chip components, which are surrounded by non-wettable component bodies. Main is the second wave (Figure 1) and is a smooth wave that prevents bridging. The smart wave is located over the main wave and produces turbulence, which may be beneficial for through-hole penetration.
Nitrogen. Nitrogen may be helpful to support flux activity during the separation of the board from the solder wave. During this separation process, the solder should stay on the joints and not in between joints. Bridging occurs because of solder oxide formation at this stage. Solder oxides are formed due to lack of flux activity and the presence of air. By applying nitrogen at that stage, it can displace the air and assist in better drainage conditions due to reduced oxides.
Cooling. As soon as the board leaves the wave, the solder joints cool rapidly at a rate of -10° to -15°C/s. Heat from the solder joints is absorbed by the component leads and the board’s copper traces/layers, resulting in rapid joint solidification. Here, the cooling system will not affect the microstructure of the solder joints, but can be used to reduce the board temperature for handling purposes.
Good wave soldering machines should have proper control and produce repeatable temperatures to ensure the profile is the same for all boards. To monitor the repeatability of a machine, SPC software is used. This helps monitor several machine parameters, detect process shifts, and recommend preventive maintenance. This way, the user can be sure all components and joints reach correct soldering temperatures.
Ursula Marquez de Tino, Ph.D. is a process and research engineer at Vitronics Soltec, based in the Unovis SMT Lab (vitronics-soltec.com); umarquez@vsww.com. Her column appears monthly.
