Electronic Component Rework, It s a Small World and Getting Smaller Mark J. Walz

Transcription

1 Electronic Component Rework, It s a Small World and Getting Smaller Mark J. Walz With the continuing decrease in electronic product sizes, micro surface mount devices (µsmds) are being applied to an ever-increasing spectrum of new products. Micro SMD components are Wafer Level Chip Scale Packages where the package size equals the die size and there is no underfill material or interposer between the silicon IC and the printed circuit board. These components are typically less than 3 mm x 3 mm (.118 x.118 ) in cross section. No matter how reliable current manufacturing processes become, there is always some cause for unexpected failure or the unanticipated need for engineering changes to circuit board design that requires rework of a component. When µsmds are used in product designs, manufacturing equipment must be more precise, repeatable and more carefully calibrated. Tooling must accommodate the light weight and small size characteristics of these components. As a consequence, rework has become a significant manufacturing challenge. This article will review semi-automatic rework system capabilities, innovative tooling options and some new techniques applied to small component rework. Materials For this demonstration, NSC s 5-bump µsmd, bpa05gab, with a 0.5 mm (.02 inch) pitch was used. This µsmd is 1.11 mm (.044 ) x 0.93 mm (.037 ) and 0.85 mm (.033 ) thick. The eutectic solder bump size is mm ( inch) diameter. A 75 mm (3.0 ) x 79 mm (3.1 ) x 1.6 mm (0.062 ) thick evaluation board was used for process development. Tooling and Equipment Recently developed tooling and techniques were applied that were aimed at adapting to the special rework challenges that very small components required. For the rework equipment, a widely used semi-automated rework system was selected. The equipment had the placement accuracy and semiautomated processing capabilities that are required for small component rework [1]. Several features that were particularly useful for successfully reworking µsmds included; precision component placement and alignment capabilities, regulation of top heater airflow rates to low levels, and pickup / placement motion control that is independent of top heater nozzle motion. In order to exploit these capabilities for reworking very small components, specialized tooling and custom rework process sequences were developed specifically designed for small components. [1] Naugler D., Summit 1100/1100HR Placement Capability, 1999, SRT Publication. 1

2 The tooling used for µsmd rework included a heater nozzle that directs heated airflow onto a very small focal point. Interchangeable nozzle heating tip inserts provided a range of possible square tip insert sizes from 2 to 7.5 mm ( inch). For the µsmd reworked in this article, a 3.2 mm (0.126 inch) square nozzle tip and 0.81 mm (.032 inch) diameter micro needle pickup tip were used. The pickup tube used a Luer Lock taper that allowed for the quick exchange of different sizes of blunt end syringe needle pickup tips. The pickup 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. In addition, the rework process was automated to allow the software to control the sequence of process steps as well as the activation of vacuum or positive air pressure within the pickup tube. The heating nozzle in Figure 1 is designed to augment heated airflow control. An adjustable vent valve allows the process engineer to reduce Vent Ring numbered position settings from 0 to 4. airflow rates by rotating the vent ring to one of five numerical detent positions between fully closed and fully open. This action alters internal nozzle air pressure and is used to adjust exit air velocity from the nozzle tip. Figure 1 Adjustable Airflow Heating Nozzle forced hot air. Airflow control features on most rework systems provide rate controls designed for reworking larger components. The vent valve enables the process engineer to adjust airflow to a lower flow rate that maximizes heating efficiency while ensuring that the component will not shift position from the impact of Rework Process Sequences In order to rework µsmds, several steps are required that include; development of a rework thermal process, removal of the defective or misplaced component, preparation of the rework site on the circuit board, and alignment, placement and reflow of the new component. Each aspect of this process will be examined with special attention to the new techniques used to rework these small, lightweight µsmds. 2

3 Rework Thermal Process Development Many semi-automatic rework systems rely on the software for automated thermal process development [2]. Using this technique, a 5-mil thermocouple (TC) is typically placed beneath a part, such as a BGA225. With the thermocouple in touch with the solder balls, a desired solder reflow temperature profile can be developed for application in the removal or reattachment of the BGA. In order for this method to be applied to µsmds, an alternative non-destructive instrumentation method was needed. The parts are too small to place a 5-mil thermocouple beneath the µsmd, and drilling the board beneath the µsmd to epoxy a thermocouple would ruin the board. A thermal process development technique was applied to the µsmd evaluation board that takes advantage of the negligible mass of µsmds. The low weight of µsmds represents a very small heat sink during convective hot air heating. In order to simulate a rework site with an attached component, four layers of 3-mil Kapton tape were used to cover a high response rate thermocouple junction at a bare µsmd site [3, 4]. This foil thermocouple had a junction thickness of only mm ( inch). Shown in Figure 2, this technique was previously correlated against a standard 5- mil thermocouple cemented in a drilled hole beneath a µsmd [5]. 1 mm Thermocouple #3 Junction Kapton Tape Layers Rework Site Thermocouple #3, at the rework site, was placed beneath four layers of Kapton tape while thermocouple #4 was affixed to the circuit board beneath one layer of tape, approximately 50 mm (2 inches) from the rework site. (Thermocouples #1 & #2 are reserved by the rework system for top and bottom heater temperature monitoring.) The micro nozzle is centered directly over the rework site to heat the rework area and profiling thermocouple. Figure 2 Thermal Profiling TC Location When setting up a thermal process, the board and rework site must be protected from thermal stress and delamination due to overheating. The site temperature can readily be maintained by the software to below 225 C in order to avoid die cracking or delamination [6]. Whenever possible, adjacent site solder joints should be held below 160 C to avoid the risk of undesirable secondary reflow or intermetallic growth within adjacent site solder. This temperature was monitored by thermocouple #4 and controlled by the software. [2] D. Naugler, Thermal Process Development for Rework Using Auto Profile Software, Proceedings of NEPCON West 2000, Anaheim, CA, Volume 1, p , February [3] Kapton is a high temperature polyimide film tape and is a registered trademark of the DuPont Company [4] Omega type SA-1-SC with self adhesive removed [5] This technique was correlated in prior work by comparing the thermocouple responses of a 5-mil TC epoxied in a hole drilled in the board beneath a µsmd and comparing the TC response to the.0005 foil TC taped to a bare site beneath differing layers of Kapton tape. [6] Parvez M.S. Patel and K. Srihari, PhD., Process Overview Rework of Chip Scale Packages, Department of System Science and Industrial Engineering, State University of New York, December

4 Consistent with these requirements, the thermal process graph of Figure 3 shows the desired time intervals and temperature set points. The software will use thermocouple #3 feedback to control top heater power and achieve the desired solder temperature profile (red line) established in the process screen. The other thermocouple will be used to independently monitor and control boardconditioning temperature (blue line) with the automatic regulation of bottom heater power. The software uses temperature feedback from thermocouple #4 to regulate bottom heater power to achieve an initial board conditioning temperature of 105 o C. Preheat Solder Temperature Reflow Post Reflow T&Ta Training Board BPA05 Micro_Reflow Site Bottom heater power is regulated to whatever levels are required to condition the board to an initial temperature of 105 o C while not allowing bottom heater air temperature to exceed 350 o C (brown line). Board Temperature TC3 -Solidify Micro After the software senses that the board has reached a temperature of 105 o Figure 3 Thermal profile process temperature targets C, the Preheat Phase begins. Top heating will ramp the site to 165 o C while the bottom heater ramps the board to 150 o C. It is important that this step allow for flux activation at a temperature and time sufficient for cleaning, but not be excessive. The 150 o C board temperature is higher than might be used when reworking larger components. It is used in this application for two reasons. It reduces the demand on top heating of the rework site, and it reduces top heater peak temperature required to reflow the site. The higher board temperature can be used only because of the accurate control capability of the rework system software. This will be demonstrated later in this article to be controlled within ± 3 o C throughout the rework cycle. The software uses TC #3 to monitor site temperature and regulate top heater power to achieve the desired solder thermal profile (red line). Power settings to the top heater are learned by the software so that the identical settings of power over time can be applied for repeatable rework processing of this site on additional boards. The yellow reference line represents liquidus of the eutectic solder and the green line, at 300 o C, represents an upper limit placed on top heater air temperature. This limit, as well as the 350 o C limit specified for the bottom heater, are user specified and are designed to protect both heaters from burnout. Following Preheat, the Reflow Phase begins. At this point the board temperature will be maintained at 150 o C while the µsmd will be raised to 205 o C and held for 5 seconds. The maximum rate of 4

5 temperature change (ramp rate) for the µsmd should be limited to no more than 2 C/sec. The peak solder joint temperature for typical tin-lead solder should reach 205 to 215 C in order to achieve good wetting of the solder to pads. The profile should dwell at reflow temperature long enough so that the solder remains above liquidus for seconds. The Post Reflow Phase will then cool the board and site so that solder solidification can occur. Two key capabilities were enabled on the process screen of Figure 5 that were used for µsmds; Use Low Airflow and Use External Control TCs: Bottom:. The Low Airflow feature minimized top heater airflow rate. In addition, the heating nozzle vent valve had to be opened 50%, which corresponded to position 2 on the valve. From prior work with this size µsmd, this vent position was determined to be necessary in order to further reduce airflow rate and prevent µsmd movement during heating. The External Control TCs: Bottom: feature assigned TC #4 as the control thermocouple for board temperature. Checking the Learn Profile button on the screen was used to start the automated thermal development process. Figure 4 shows the resultant thermal profiles that were achieved. usmd Reflow Site Auto Profile Bottom Heater Control TC Top Heater Control TC Temp ( o C) Site TC External Control TC of Board Temperature Time (seconds) USMDRE~1.LOG :02:34 PM USMD Development BPA05 USMD_Reflow Site Figure 4 Temperature and time plots of achieved thermal profile In this graph the rework Site (green line) represents thermocouple #3 that was placed beneath four layers of Kapton tape. The dotted pink line represents liquidus of the solder (183 o C). Analysis of the data showed time above liquidus to be 78 seconds. A peak temperature of 205 o C was reached at the site while board temperature was maintained between 147 and 153 o C by thermocouple #4. The temperatures achieved at the site are well within limits specified in NSC s Application Note 1112 for their µsmds. This thermal process can now be applied to component removals and replacements. 5

6 µsmd Removal Removal of a defective component was accomplished by heating the µsmd with the learned thermal profile while using a removal sequence that minimizes pick pressure applied during removal. Minimizing the pressure applied for pick and removal enables the µsmd to be removed with a minimum of disruption to the solder on the site pads, making it easier to remove the residual solder. The rework system first locates the top of the part with the pickup tube before heating begins. The rework system records this pick position before the part is heated to reflow temperature. After achieving reflow temperature, the micro tip is moved to the stored pick location at the top of the component and a vacuum is applied in order to lift the component from the site. The sequence of mechanical steps and the application of heat are controlled by the software. Site Preparation A new site dressing technique was applied after removal of the component. Micro SMD sites are too small to use conventional solder wick or vacuum scavenging approaches to remove residual solder. Solder wick can damage delicate site pads or cause them to be pulled from the board when the temperature is not uniformly above reflow or from friction between the solder wick and pad. Vacuum scavenging effectiveness is limited because the tips are typically larger that the µsmd sites to be cleaned. A residual solder removal technique was developed using copper scavenger coupons. These thin coupons are made to the precise dimensions of the site, using as much additional space as adjacent components allow. This provides as much copper surface area as possible for solder attachment. The copper coupons are picked, dipped in a flux tray, aligned, placed, reflowed and removed using a semi-automated sequence of process steps developed for this rework system. Thorough heating of the copper scavenger coupon eliminates the potential for pulling site pads due to inadequate temperature. Using the precise pick and removal capability of the rework system avoids frictional contact forces between the copper coupon and site. The reflowed solder wicks to the coupon and is then removed with the pad and discarded. Using the process sequence designed for this purpose, a copper coupon measuring 1.42 mm (0.056 inch) square was dipped in a flux tray, placed on the site, reflowed and removed. The heating nozzle vent valve was fully opened to position 4 in order to reduce the airflow rate and prevent movement of the lightweight copper coupon during heating. A separate thermal process had to be learned for this valve 1 mm setting that produced a much different heating cycle as compared to the 50% open position used for µsmd removal and replacement. After copper coupon removal, the site was quite clean and the solder thoroughly wicked to the coupon shown in Figure 5. Site pads remained tinned with a thin layer of solder and showed no evidence of pad or site damage. Figure 5 Underside of copper coupon after solder removal 6

7 µsmd Placement Applying the developed thermal process to the placement and reflow of a 5-bump µsmd was then performed on the cleaned site. The µsmd was picked from an ESD cushioning pad on the pick nest of the X-Y axes positioning table. A slight amount of tacky flux was applied to the µsmd by using a mm (.003 inch) deep cushioned flux tray. The flux tray had embedded cushioned rubber pads to prevent distortion of the tiny solder bumps during the dip process. Use of a cushioned flux tray was necessary because the rework system applies roughly 50 grams of force in order to sense that it has made contact between the µsmd and flux tray. The 50 grams of contact force equates to a high contact pressure that can distort the tiny solder bumps when a typical metal flux tray is used. For this same reason, picking parts required an ESD rubber pad to be placed in the area of the pick location. The ESD rubber pad provides a cushioned pick surface electrically grounded to the rework system. Placement of the µsmd was the next step. In all cases a 40 X magnification and up/down viewing capability of the rework system was used to align components to site pads. The up view of Figure 6 shows the outline of the 3.2 mm square heating nozzle tip and µsmd held by the pickup tube micro needle, while the down view facilitates alignment of solder bumps to pads. The 1 mm pickup tube is then lowered to precisely place the µsmd on the site. Following µsmd placement on the site, the heating process that was developed for this purpose was used to reflow the site. Following reflow, transmission x-ray inspection and analysis showed evidence of good pad wetting and excellent alignment of pads to solder balls. Figure 6 Aligning solder balls to site An effort was then made to deliberately misalign µsmd solder bumps to site pads by approximately 50% (simultaneous up and down views) in order to examine the self-aligning capability that is claimed in the previously referenced NSC applications note. This self-aligning ability can be observed with larger BGAs where the surface tension of the solder can help the BGA to seat more accurately onto the site pads. Transmission x-ray inspection was used after misplacement of a µsmd, and prior to solder reflow in order to confirm the 50% misalignment between solder bumps and pads. After reflow, the transmission x-ray image of Figure 7 was obtained. The false color image replaced shades of gray with high contrast colors in order to make it easier to determine the quality of the solder joint. The image shows good wetting of the solder to pads, no voids and excellent alignment of solder balls to pads. It is clear that Figure 7 False color enhancement of post-reflow X-ray inspection 7 (high angle, rotated view)

8 solder surface tension does have the ability to correct for a significant degree of µsmd misalignment. Conclusions In the world of shrinking electronic product designs, the difficulty of reworking very small parts can be readily overcome with a high degree of confidence and success. Applying new techniques and tools with existing rework systems can yield successful rework of Wafer Level Chip Scale Packages and other very small parts. Adaptation of existing equipment and software must take into account the special considerations such small sizes require. The ability to adjust and control very low airflow rates are essential to preventing the movement of small, lightweight components after alignment and during heating. It is advisable to have a means of numerically adjusting airflow rates in order to maximize repeatability of the rework thermal process. Using a cushioned flux tray and soft pick pad can prevent accidental damage to very small and soft solder bumps when dipping parts in flux and picking parts for placement. When preparing sites for new components, care must be taken to avoid damage to the small and delicate board sites. For this purpose, residual site solder can be precisely and safely removed by using copper scavenger coupons. In spite of their small size, micro SMDs exhibit the same self-aligning characteristics as their larger cousins. Solder surface tension upon reflow will enable the micro SMD to shift position and correct for fairly substantial misalignments between solder bumps and pads. About the Author Mark Walz is the founder of Training & Tooling Associates, Inc., a firm specializing in software training, applications engineering and tooling associated with circuit board rework. Mr. Walz received his BS in Pre-Medicine from Brooklyn Polytechnic Institute and his MS in Biomedical Engineering from Worcester Polytechnic Institute. He has over 20 years of experience in engineering and new product development and is the holder of several patents. Training & Tooling Associates 63 Colburn Street Northborough, MA USA mjwalz@trainandtool.com 8