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Martin CSP Solution |
published in epp 02/2007 |
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| It is quite common for a rework expert company to deliver to its clients efficient solutions for the use in their repair department. However, it is not standard for such specialists to go and eventually deliver an initial assembly solution for brand-new components which cannot be sufficiently soldered because of wetting problems at the solder joints. This was roughly the situation when a large global player approached the small specialist firm, Martin, with just a handful of boards and CSP specimens that could not safely be soldered, and hoped that these qualified folks would solve the pressing issue. |
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| In this particular case the focus was laid on a device, a CSP (chip scale package) with the size of 10 x 13 mm, a pitch gauge of 1.0 mm, and 64 solder balls measuring 0.4 mm in diameter for the I/Os. This typical new-generation SMD (figure 1) has apparently been used by many companies worldwide in Europe, the U.S. and Asia. |
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| And we heard that the component was about to be implemented in first board assemblies in the pre-production area or in prototype products at some companies. The experience of these users has been all but encouraging though: after the lead-free soldering procedure the part did not rest evenly on the board assemblies, instead it formed a slight angle with the flat PCB surface. First and foremost, the solder joints were not properly reflowed due to the poor wetting behavior of the solder alloy used for the CSP balls. |
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Figure 1: The 64-I/Os CSP which created the trouble on the shop floor |
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Figure 2: X-ray image of the insufficiently soldered and irregularly positioned CSP. Note the elliptical outline of the soldered balls and the white spots (relatively large voids) in them |
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The X-ray analysis (figure 2) provided afterwards by us and other soldering specialists even revealed that too many und too large voids (incorporated air) were hidden in the solder joints, with all the associated problems for the reliable and secure functioning of the board assemblies months later in the field. In a nutshell: although this soldering result appeared to be just a "cosmetic" problem at first, this was certainly not the product quality expected by a manufacturing company. By trial and error some users, especially in Asia, tried to compensate for these irregularly inclined components, for example by putting some tiny weight on the part during the soldering process. |
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But obviously this could only be a provisional answer to a critical manufacturing dilemma. As already mentioned, it was initially just a visual annoyance not showing any electrical dysfunction so far. However, functional disorder of these board assemblies could probably be expected at customers' sites in the course of time.
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In search of the miraculous |
Using the client's existing board assemblies, PCBs and components, we - the Martin soldering experts - went on to clarify the original source of the soldering difficulties step-by-step. The main question was: did the PCB, the solder alloy or the solder balls create the trouble? Also paste print, the component placement and flux media used can severely contribute to such an unwanted effect. As we know, there is a cornucopia of possible reasons which can disturb the proper soldering process flow and every single element has therefore to be carefully examined.
Troubleshooting of this kind requires profound knowledge of, and expertise in, the assembly process and especially of all the technology features that are acquainted with soldering procedures. This type of research is really a time-consuming task, but nevertheless a necessity when serious shop-floor issues have to be solved. The job is also evidence of substantial trust from a patron relying on a long-standing and successful partnership with a small specialist firm and its high competency in this discipline. It is by no means the normal task of a rework expert to find production-related defect sources, so all the better that we - the Martin folks - were chosen to do this and successfully provided the solution at the end of the day.
However, first there was, of course, a great deal of work to do. It is known that in some geographic regions the application of a more aggressive flux is often the remedy for wetting problems. But this was not the client's and our choice; we were forced to find the real reason. Since the quality and reliability of semiconductor components have remarkably become better over the last two decades, our first premise was: the device should be good, but maybe the PCB isn't sufficient. In the course of the work, we went through the entire catalog of possible measures: changing the soldering profile in its main parameters such as the volume of air stream, the temperature and the dwell time. We started with the normal soldering temperature profile for the usual lead-free SAC (tin-silver-copper) solder paste and then tested all possible variations. We also applied different flux creams, cleaned the solder balls and PCB pads with special glass-fiber pens. The latter is definitely not the option of choice on a shop floor, but was helpful in our investigation.
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Throw 'em out: inadequate solder balls |
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Then we moved our focus to the device in question, in particular to its solder balls. We removed them and applied new ones carefully. And indeed, the problem was no longer present. This was finally the solution we presented to our customer for his application of these CSPs in regular products - and it was accepted.
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| We also slightly modified the soldering tool used for the reballing procedure, in order to provide the hot air stream even more optimally to the CSP balls for reflowing. From our portfolio of consumables we applied new BGA/CSP solder spheres (balls) made from Sn96.5Ag3Cu0.5 which form solid, reliable and wetting-proven solder contacts. This was verified afterwards by X-ray analysis (figure 3). The images showed the sharp contour of well-reflowed solder balls and revealed that there were good soldering procedures provided. Plus no irregular level variations between the PCB surface and the CSP could be seen, indicating that the small, constant gap required between them was maintained. The voids in the new reflowed solder balls were also virtually eliminated. This research was prepared and conducted with the kind support of the German ZVE organization (Development Center for Interconnection Technologies, a society of the Fraunhofer IZM research institute) by Dr. Sonja Wege. |
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Figure 3: X-ray image of the correctly soldered and regularly positioned CSP. Note the exact circular outline of the soldered balls, and that there are practically no white spots (voids) |
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For the soldering task we used the latest version of our worldwide established rework station, the Auto Vision Expert 09.5, together with our proven reballing system. (Please see the side bar in this article for a more detailed description of the equipment, its functionality and the easy-to-understand workflow.) The entire process can be completed on the Auto Vision Expert (figure 4): removal of the old and inadequate solder balls, reballing, generation of the correct soldering profile and the precise reflow according to this profile.
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Figure 4. The complete Auto Vision Expert system encompassing the base unit with the moving camera arm and the PCB underheater, the control module with LCD (left), the monitor (right) with the interactive program Easy-Solde -05, and the quick change magazine for placement nozzles and soldering tools (in the foreground) |
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| The reballing system (figure 5), consisting of a metallic frame (stencil) with a grid of holes imaging the ball array and a miniature oven, can be used with all our Martin rework units. The process itself is relatively easy to accomplish. |
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The reballing set and its constituents - the Mini Oven, the device and on top of it the mask for the solder spheres (balls), operated by the Hot Air Pen according to the reflow profile generated and stored in the Auto Vision Expert system |
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The cleaned and Flux Pen-coated device is placed in the tiny furnace and afterwards covered by the small grid mask. Then the solder spheres are distributed over the mask, with excess spheres transferred back into their glass container. After this, the closed Mini Oven is placed on the reversed Hot Air Pen (which, in turn, is controlled by the rework station) - and the suitable soldering program can be started from the Auto Vision Expert.
We can see from this solution that our rework station is an effective and versatile work place, featuring a wealth of useful and practical tools and functions for the entire repair-task palette, as well as for special jobs in the pre-high-volume stage. And not to be overlooked: all tasks can be easily, precisely and repeatably provided by the regular production staff without any extensive operator training. These assets make the Auto Vision Expert 09.5 the ideal system for such demanding jobs in today's electronics manufacturing.
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Safe rework with precision and intuition -
whether lead-free or lead-bearing |
Our compact and cost-efficient benchtop rework system Expert 09.5 is suitable for assembly dimensions from just a few mm to up to 500 mm in length, and features a flexible PCB support and adjustable area underheater of the IR or IRF type. The adaptation of the workstation to the board and component size is just a matter of seconds; what might have to be changed are the placement nozzle, the soldering tool and the camera lens. Featuring the patented Martin Auto Vision technology, a precision camera (vision) system monitors and controls exactly the placing of the parts, so even highly demanding repair actions can be easily managed. The system employs the Martin Zero Risk Rework heating technology to fit into the extremely narrow lead-free process window, without any risk to product quality or reliability.
The thermodynamic soldering concept is based on just two essential elements: the background heat is delivered to the board by the fast-acting IR area underheater, and on the topside an application-specific soldering tool supplies the patented Precision Hot Air system for secure rework. This strategy provides half the energy through each heating circuit and avoids board warping and overheating. The heat energy which is delivered at the top to the solder joints follows a very precisely controlled (± 1%) hot-air stream, with no calibration necessary by the users. In order to comply with ISO 9000 and other quality-assurance strategies, the Soldering Pen can be cost effectively exchanged or re-certified using the set Easy Cal III. The temperature is evenly distributed across a component within ± 4K, resulting in virtually simultaneous reflow of all solder joints of a component. The proven asymmetric airflow avoids overheating the center of the components. The Rapid IR underheater transfers the background energy into the PCB within a few seconds without exceeding the maximum permitted ramp rate of 3.5K/sec. This allows the reduction of hot-air temperatures for the top convection heating and provides for very smooth processing, plus it delivers the resulting soldering profiles much faster than before. The area underheaters self-calibrate each time they are switched on.
Easy Solder 05 is the intelligent application software for the PC-controlled Expert 09.5 system, meeting the requirements on clarity, ease-of-use and faster processing, and aimed at the specific rework steps with more visual and automated support. For the control and traceability of work quality, the optional report function collects and records all relevant data and images in the background. The easy-to-provide setup consists of three brief steps: selection of board, choice of SMD, decision on the type of process - and then start. The program automatically establishes the process parameters such as time cycles, temperatures and airflow rates in the different profile sections. The Auto Profiler and the Quick Profiler potentiate the Expert system to easily generate profiles for specific conditions. Special processes and uncomplicated adaptations are ensured by the five rules of soldering as defined by our company and implemented into our rework equipment.
The placement of the parts occurs also highly accurately by means of a vision system, monitoring and controlling the fault-free automatic placement process from start to finish. Asymmetric SMDs, too, are measured by the Auto Vision Placer and placed with the required offset. The Auto Vision Placer receives the coordinates of the corner points of the component and the placement positions by just some mouse-click confirmation steps. This is easy due to the magnified board area, plus the highly developed software leading users through the required steps effortlessly. The system automatically calculates the precise movement coordinates, and with the help of the vision system all axes motions are continuously controlled. As a result, the placement operation does not primarily depend on mechanical accuracy, but on the high resolution of the vision system.
The also used reballing system is available for CSPs, QSNs and BGAs. In the case of CSPs, the Mini Oven Set has the dimensions of 27 x 27 mm and features Capton tape and cutter for preparation. The metal frames (stencils) for the distribution of the solder spheres can have dimensions from 5 x 5 mm to up to 20 x 20 mm, and others are also possible. The BGA reballing Mini Oven features dimensions from 15 x 15 mm to up to 45 x 45 mm. Accordingly, Grid Masks can be provided, also application-specific.
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