-
- News
- Books
Featured Books
- smt007 Magazine
Latest Issues
Current IssueBox Build
One trend is to add box build and final assembly to your product offering. In this issue, we explore the opportunities and risks of adding system assembly to your service portfolio.
IPC APEX EXPO 2024 Pre-show
This month’s issue devotes its pages to a comprehensive preview of the IPC APEX EXPO 2024 event. Whether your role is technical or business, if you're new-to-the-industry or seasoned veteran, you'll find value throughout this program.
Boost Your Sales
Every part of your business can be evaluated as a process, including your sales funnel. Optimizing your selling process requires a coordinated effort between marketing and sales. In this issue, industry experts in marketing and sales offer their best advice on how to boost your sales efforts.
- Articles
- Columns
Search Console
- Links
- Events
||| MENU - smt007 Magazine
Passive Component Rework - Giving Small New Meaning
December 31, 1969 |Estimated reading time: 8 minutes
The prevalence of small passive components in electronic products creates a challenge when adapting rework systems for selective removal and replacement. Manufacturers semi-automate this process to streamline quality. This article reviews the efforts taken to adapt a common rework system for semi-automated 0201 removal and replacement.
By Mark J. Walz
With the continuing decrease in electronic product sizes, small passive components are being used more frequently. A fraction of improperly placed passive components present challenges in adapting existing rework systems for selective removal and replacement. Common passive components include 0603, 0402 and 0201 resistors that measure 0.02" x 0.01". Handling individual parts quickly becomes a tedious activity that gives new meaning to small component rework. Many manufacturers attempt to semi-automate the rework process to reduce variability in quality. Tooling must accommodate the light weight and small size of these components.
Tooling and Equipment
Recently developed tooling methods were aimed at adapting to the challenges of reworking passive components. A semi-automated rework system with the required precision placement capability was used.1 For this article, a 0201 was reworked on a 3"- x 3.1"- x 0.062"-thick evaluation board. To adapt system capabilities for small component rework, specialized tooling and custom-rework process sequences were developed. The tooling used for 0201 rework included a precision heater nozzle that directs heated airflow onto a small focal point. Interchangeable nozzle heating-tip inserts were available, and a 0.079" square nozzle tip and 0.015"-diameter pick-and-place needle were used.
A 0.079" square heating tip is larger than a 0201; however, there is a limit on how small the hot-gas heating tip can be and still provide a reasonable rate of heat transfer to small components. The height in which the heating tip is raised above the component during heating also impacts heat transfer efficiency. Allowing the heated gas to vent between the gap in the circuit board and heating tip improves heating speed. The downside of this approach is that it cannot be used to avoid adjacent component reflow.
A redesigned component pick-up tube was used to allow for quick exchange of different sizes of blunt-end syringe needle pick-up tips. The pick-up tube and micro-needle assembly applies a vacuum to pick parts, and a pulse of positive air pressure to aid in the release of parts during placement. The heating nozzle in Figure 1 provides a mechanism for additional regulation of the heated airflow rate. An adjustable valve allows reduced airflow rates through vent-ring rotation to one of five numerical positions. This opens or closes the valve, regulates internal nozzle air pressure and can lower exit-air velocity from the small nozzle tip.
Figure 1. Adjustable airflow heating nozzle.
Airflow control features on most rework systems provide rate controls designed for heating larger components. The vent valve enables process engineers to adjust airflow for maximum heating efficiency, while ensuring that components will not shift position from the impact of forced hot air.
Rework Process Development
Reworking 0201 components involves developing a rework thermal process; removing defective or misplaced components; preparing the rework site on the circuit board and aligning, placing and reflowing new components. Each step of this process must adjust to special size and weight requirements of 0201s.
Many semi-automatic rework systems use software for automated thermal process development.2 Most approaches require placement of a thermocouple beneath the part to provide temperature feedback and regulation. Small passive components usually are in intimate contact with the circuit board, which makes it difficult to instrument 0201s with a thermocouple. Figure 2 shows a technique that was applied to take advantage of the negligible mass and small heatsink characteristics of passive components. A 0201 site was simulated where three layers of 3-mil, high-temperature polyimide film tape were used to cover a high-response-rate thermocouple junction. The tape was cut away to limit tape coverage to the site area and TC junction. The micro nozzle will be centered directly over the 0201 site-profiling thermocouple to heat the rework area.
Figure 2. Thermal-profiling LC locations.
The board and rework site must be protected from overheating, thermal stress and delamination.3 If required, adjacent site solder joints can be held below 160°C to avoid the risk of undesirable secondary reflow or intermetallic growth within adjacent site solder. This can be accomplished by using non-solderable thermal shields (made from stainless steel or aluminum) to cover adjacent site components during hot-air heating.
Desired time intervals and temperature set points are shown in Figure 3. The software will use thermocouple #3 feedback to control top-heater power and achieve desired solder temperature profile (red line) established in the process screen. Thermocouple #4 will be used to monitor and control board-conditioning temperature (blue line) with the automatic regulation of bottom-heater power. Bottom-heater power is regulated automatically to condition the board to an initial temperature of 100°C, while not allowing bottom-heater air temperature to exceed 325°C (brown line).
Figure 3. Thermal-profile temperature targets.
Once the board has reached 100°C, preheat phase begins. Top heating will ramp the site to 170°C, while the bottom heater ramps the board to 150°C. It is important that this step allow for flux activation at a temperature and time sufficient for cleaning, but not excessive. The 150°C board temperature is higher than that used when reworking larger components. It is used in this application to reduce demand on top heating of the rework site, and to reduce top-heater peak temperature needed to reflow the site.
The yellow reference line represents liquidus of the eutectic solder; and the brown line, at 350°C, represents an upper limit placed on top-heater air temperature. This limit, and the 350°C limit specified for the bottom heater, are designed to protect both heaters from burnout.
There typically is a time lag between target temperatures and what can be achieved with small passive components. This is attributed to the limited heat transfer rate that can be attained with low-gas flow rates. These low rates must be used to avoid component movement prior to reflow. However, peak solder-joint temperature for typical tin/lead solder needs to reach 205° to 215°C to achieve good solder-to-pad wetting. The profile should dwell at reflow temperatures long enough so that solder remains above liquidus for 45 to 90 seconds.
The rework system’s low-airflow feature minimizes top-heater airflow rate. The heating nozzle vent valve also had to be opened. From prior work with 0201s, this was necessary to reduce airflow rate further and prevent 0201 movement during heating. This optimal setting also is dependent on the height between the circuit board and the heating tip.
Analysis of data collected during the heating process showed that a time above liquidus (TAL) of 89 seconds was achieved. A peak temperature of 209°C was reached at the site. This thermal process met our temperature objectives and can be applied to 0201 removals and replacements.
0201 Removal
Removing a defective 0201 was accomplished by heating the 0201 with the learned thermal profile and using a customized removal sequence designed to accommodate the new tooling. Removing 0201 components with small-diameter pick needles can be problematic. Small needle diameters do not produce as much vacuum pick force. With limited pick force, reliably overcoming surface tension of molten solders can be difficult. An alternative method for this includes using an oversized pick needle to “scavenge” the defective 0201 from the circuit board. Placing a metal filter screen in the pick-needle hub can trap removed parts.
Site Preparation
The 0201 site was left with residual solder to help attach new components. However, residual solder volume usually is insufficient for a good solder joint and may be removed if desired. To augment residual solder volume, an upper nest was used with a clamshell fixture to locate replacement 0201s over precision solder-paste stencil apertures. Locating part-nest designs must accommodate the irregular shape of metallized ends of passive components. “Dog-bone” shaped nest holes are laser-cut to provide corner relief in nests. Some nests are located above the stencil (Figure 4). Production processing time is minimized through the use of multiple nest holes to locate and apply solder paste simultaneously to several 0201 components.
Figure 4. 0201 nest and stencil apertures.
Nests were photo etched, while laser-cut and electropolished stencil apertures provided the precision and smooth surface finish required for good solder-paste release. Aperture ratios carefully were considered for reliable solder-paste release. After applying solder paste with a squeegee to fill the stencil apertures, individual 0201s can be picked from the nest for alignment and placement on a site. Pick-needle selection should be a compromise between using the largest possible internal diameter to maximize pick force, while providing the greatest flat-pick surface area without interference from 0201 metallized ends.
Placement and Reflow
The 0201 was picked from the solder-paste stencil nest located on the rework system-positioning table. Figure 5 shows simultaneous up/down vision alignment and placement of the component onto the site. Following placement, the heating process was used for reflow. A suitable solder joint was achieved without displacing the 0201 with the force from the heated airflow. Solder joint and placement alignment quality were verified with optical and transmission X-ray inspection.
Figure 5. Up/down video alignment of 0201 to site. A 0.015" x OD x 0.010" ID-pick needle shown above in a 2-mm heating tip.
null
Conclusion
With the continual progression of shrinking electronic product designs, reworking difficulty for small parts can be overcome with a high degree of confidence and success. Manufacturing engineers and technicians must adapt their skills to an environment that gives “small” a new meaning. The ability to adjust and control low airflow rates is essential to prevent movement of small, lightweight components after alignment and during heating. It is advisable to have a means of numerically adjusting airflow rates to maximize repeatability of production rework processes.
Residual site solder must be augmented with solder paste, obtaining adequate solder volume for replacement component attachments. Careful consideration of solder-paste stencil and nest designs is required to accommodate the irregular shape and limited tolerance control of the metallized ends of passive components. While it is possible to manually rework 0201 components, significant volume of this type of rework can become tedious. Good equipment maintenance and calibration is mandatory. However, adapting existing semi-automated rework equipment and appropriate tooling can provide improved production consistency and reduce required technician patience and skill.
References
- Naugler D., “Summit 1100/1100HR Placement Capability”, 1999, SRT Publication.
- D. Naugler, “Thermal Process Development for Rework Using Auto Profile Software”, Proceedings of NEPCON West 2000, Anaheim, CA, Volume 1, p. 20-29, February 2000.
- Parvez M.S. Patel and K. Srihari, Ph.D.., “Process Overview - Rework of Chip Scale Packages”, Department of System Science and Industrial Engineering, State University of New York, December 1998.
Mark J. Walz, president, Training & Tooling Associates, may be contacted at (508) 393-6781; e-mail: mjwalz@attglobal.net.