CircuiTree - February 2009 - (Page 28)

The Big Deal Over Fine Pitch Assembly eral key considerations and carefully applied steps. The printing mechanism should also be carefully planned out, keeping in mind the squeegee pressure, paste type, as well as cycle time for dispensing. Stencil printers need to be calibrated to every beginning of the assembly shift unless it is autocalibrated. Residues of previous solder paste stuck to a stencil’s top and bottom sides must be cleaned as often as every two hours when printing is being performed. Squeegee blades must be cleaned every two to three cycles in the case of regular assembly, but for ﬁne pitch devices and BGAs, it should be cleaned after every cycle. Also, the amount of stencil usage is often overlooked. The same stencil, when used a few hundred times on ﬁne pitch SM devices, should be a candidate for replacement. Also, stencil printing must be performed by a printer designed for ﬁne pitch devices (Figure 4). With ﬁne pitch devices being so close to each other and squeegee pressure being applied so often, the distance between pads may begin to widen. As a result, more solder paste than necessary is applied where it’s not required. Sometimes there are cracks that start appearing between ﬁne pitch SMD pads, thereby needing replacement. Also, the solder mask must be precisely applied on the board populated with ﬁne pitch devices. The entire pad of a ﬁne pitch SMD must be exposed so that a complete pad surface is available for assembly. The pad should not be covered by any part of the solder mask or by silk screen. If the silkscreen is not properly placed and an SMD pad is covered, even partially, then that portion of the pad will not properly solder to the ﬁne pitch device. Nonstandard or weak solder joints are the result. To avoid this problematic area, it’s important to clip protruding portions as part of the silkscreen printing process. Laser solder jet printing, a brand new technology, is providing a more effective method of dispensing the solder paste directly on the SMD pads compared to traditionally using the stencil. It not only improves the process control, which is quite crucial for ﬁne pitch SMD assembly, but also can be used very effectively when a higher density of components is involved. Laser printing deposits solder paste using laserjet technology, which can ﬁne tune the paste deposition in volume, area coverage, or pad shape on the ﬂy practically without touching the PCB. If one side of the board is mostly popu- Figure 4 Stencil Printing Performed Using a Printer Designed for Fine Pitch Devices populated with ﬁne pitch devices. But hot air solder level (HASL) should be avoided because it is not as precisely controlled a process on the pad compared to gold or silver. Because it is not, the uniformity of deposition is not present, thereby making ﬁne pitch assembly less than optimal. While immersion gold or silver is ideally suited for ﬁne pitch, OSP should not be overlooked because it also produces an extremely ﬂat surface. However, regardless of the ﬁnish—gold, silver, or OSP— there are various tradeoffs associated with each. As shown in Table 1, those are shelf life, cost, reﬂow cycles, and solder joint ﬂatness. Immersion silver and gold are expensive metal alloys, thereby increasing the cost of producing the bare boards. At production levels, these ﬁnishes could cost 5 to 10 percent extra, depending on the amount of exposed surfaces. Therefore, it’s best from a cost/performance point of view to select a ﬁnish that can be cost justiﬁed in a particular end product. As for shelf life, HASL has about 18 months, but OSP has only 6 months. Immersion silver has a shelf life ranging from 12 to 16 months, while immersion gold is the most durable at 24 months. OSP cannot undergo more than two to three reﬂow cycles. If the need for rework continues, then the SMT pads on the OSP ﬁnish begins peeling off. Thus, OSP is not the best ﬁnish for rework. On the other 28 February 2009 • circuitree.com hand, immersion silver or gold can undergo six to eight reﬂow cycles. Using immersion gold or immersion silver results in a ﬂatter PCB surface ﬁnish. Consequently, the ﬂatter PCB surface ﬁnish is considerably more conducive to a perfect ﬁne pitch PCB assembly compared to an HASL ﬁnish. Also, these ﬁnishes withstand higher temperatures and there is a much less likelihood of the pads being peeled off from the board surface when it is exposed multiple times to higher temperature cycles. Assembly Stencil design plays a vital role in ﬁne pitch assembly, which means that its thickness and proper deﬁnition aperture rank high in importance for perfect assembly. It’s also important to deﬁne the correlation between the board itself and the aperture sizes for the SMD pads. Should the aperture be opened at a 1:l ratio in respect to the size of SMD pad? Or should it be greater or less than a 1:1 ratio? This depends on the amount of solder to be deposited on every pad of the ﬁne pitch SMD. It is not a good idea to apply too much solder paste because it will create the possibility of shorts or bridging (Figure 3) between different SMD pads. At the same time, too little solder paste should not be applied in the printing process because it will create the possibility of opens within solder joints or perhaps cold solders. Properly deﬁning the aperture requires sev- http://www.circuitree.com

Table of Contents Feed for the Digital Edition of CircuiTree - February 2009

CircuiTree - February 2009 Contents Lead Wire The Wire Tech Talk Flexible Thinking Are Better PCB Design Tools and Manufacturing Enough to Stay Competitive? The Strategist Fein-Lines The Big Deal Over Fine Pitch Assembly Asian Section View From the Middle Market Outlook Technical Product Spotlights Classified Ads Upcoming Events Ad Index

CircuiTree - February 2009

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