July, 2012 Continued from previous page
component spacing, pin toe and heel, pin width, and overall assembly review. Any one of these items can delay a schedule or cause a board to need rework. Many of these issues can result in scrapping an entire fab- ricated lot. Pin pitch mistakes typically are
made during the conversion from English to metric or vice versa. The other error made is not identifying
Package PCB Package PCB Package Even PCB
The Texas Instruments MicroStar BGA spec illustrates how to create an even solder ball for the strongest BGA joint.
the correct controlling dimensions. The majority of mechanical drawings include the controlling dimensions. Pin pitch is a cumulative error
and rarely is a problem with low pin- count items. The more pins a device contains, the larger the cumulative error becomes. For example, 0.5mm converted to mils equals 0.019685. A common mistake is to round up to
Weak
www.us-tech.com The Reality of Design for Assembly
.020 or 20 mil, with the difference being 0.000315. The difference is not enough in a 6-pin device to cause assembly issues; however, on a 48- pin device, the difference grows to 0.007, causing the pin to no longer fit on the intended pad. Row pitch does not suffer the
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same tendency for cumulative errors except on multi-row/column compo- nents. Ball grid arrays (BGA) and connectors should be built in the original dimensions. Row pitch on quads can force a designer to use smaller pads on the corners or increase the toe while decreasing the heal of the solder joint. Typically, pin type errors are made dur- ing the component lookup or a late bill of material (BOM) change.
The manufacturer part
specs are accurate between SMT and through-hole pins. The conflict arises on mount- ing holes. Many manufactur- er specs do not indicate if the mounting pins are plated or non-plated. Press fit pins require a tighter tolerance and should be noted in the fabrication and assembly
drawings. Component spacing affects the initial placement and the effort level of rework. BGAs require room for rework or the surrounding compo- nents will need to be removed before the BGAs can be removed. Pin toe and heel are critical for a
solid solder joint. According to Texas Instruments’ solder pad recommenda- tions for surface mount devices
The MicroStar spec can be used as a guide to design the pin size for BGAs.
pad than reflow boards. A major con- cern when making the pads wider is the soldermask webbing. The solder- mask webbing between the pins pre- vents shorts.
BGA Pin Size BGA soldermask and routing on
the assembled side can destroy assembly yields. The smaller the BGA pin pitch, the more crucial the soldermask becomes. The majority of the manufacturer’s drawings do not show the actual pin diameter but rather the ball diameter. The
DFA is continuous process in
which assembly, fabrication and lay- out must all work together. Under - standing the requirements of each is key to creating a successful, smooth- ly running assembly. The layout designer plays an important role in this, acting as a mediator between the engineer, fabrication shop and assembly needs. Contact: ACD, 1250 American
Pkwy., Richardson, TX 75081 % 469-624-5153 E-mail:
noah.fenley@acdusa.com Web:
www.acdusa.com r
(SMD), “The criteria for a well- designed solder joint is based on both empirical data and reliability testing. Solder joint strength is directly relat- ed to the total solder volume. An observable solder fillet is evidence of proper wetting. Therefore, a positive solder fillet is usually specified. A joint can be described by the solder fil- lets formed between the device pins and the PCB pads.” Pin width also is a factor to con-
sider. The expected assembly process will help determine the width varia- tions for the pin width increase. Wave solder boards will need a wider
MicroStar spec can be used as a guide to design the pin size for BGAs. Some manufacturers have started using the pin size instead of the ball size. These few specs match with the MicroStar spec guidelines. Once the pad is the correct size,
the routing must be considered as well for reliable assembly. Gang routing the powers and/or grounds will cause the ball to deform. The shape change will cause shorts and opens on the BGAs. Each pin should have a via to allow a low inductive path and to prevent the ball from deforming.
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