Page 56
www.us-tech.com
July, 2012 The Reality of Design for Assembly By Noah Fenley, ACD, Richardson, TX
ful assembly. Unfortunately, this often includes an ongoing struggle to balance assembly, fabrication and layout ? these elements must work together in order for the process to run smoothly. It also is necessary to understand the requirements and limitations of each of the three ele- ments. Enter the layout designer, who strives to create a product that is easy to assemble. The easier the product is to assemble, the lower the final product cost. In order to design the best
U
assembly, the layout designer must understand the fabrication limita- tions of the components and the fab- rication shop. The layout designer is a mediator between the require- ments of the engineer, the fabrica- tion shop’s abilities and the needs of the assembly itself. DFA is more than placing com-
ponents at a safe distance from one another. Today’s fine pitch compo- nents push fabrication tolerances to their very limits. For proper footprint
DFA is continuous
process in which assembly, fabrication and layout must all work together.
development, it is crucial to under- stand the fabrication shop’s mini- mum solder mask webbing. In addi-
nderstanding the concept be - hind design for assembly (DFA) is the key to a success-
tion, copper land patterns should take soldermask webbing into account. Oversizing the width of the pin footprint should not eliminate the soldermask webbing. Soldermask ganging — combining the pins into one soldermask opening — increases the likelihood of shorts between the
exposed for testability. Adequate clearance from pins allows test probes to reach the via and allows a soldermask web. Vias also cause concern for cold
Solder bridge caused by inadequate soldermask between a pad and via on the opposite side of the board.
pins during assembly. Therefore, it is best to avoid ganging soldermask when possible. Vias currently are a major fac-
tor in testability and DFA. Vias that are too close to a pin do not allow sol- dermask webbing, and this absence will starve the solder from the pin. The paste will travel through the via and short components on the oppo- site side of the board. Vias should be
solder joints. On a multi-lamination board with four or more ground fills, a direct connect via can absorb the heat into the planes and cause cold solder joints. Inadequate sol- dermask between a pad and via on the opposite side of the board can cause a solder bridge. In the case illustrated here, the pin also suffered from solder starvation since the solder was wicked away from the pin. The solder that was wicked away caused a short fount under the connector. These types of shorts are difficult to find because components must be removed to debug. Now consider connector placement. Allow ample room for mating connection and removal. Not all connectors are a single plug-in. Placing a compo- nent too close to a connector is a common mistake. The component blocks the end user from removing
the connection. Even placing compo- nents on the edge of the board requires space. Several other connec- tors require wrenches or other tools to connect them correctly. Silkscreen is used for orienta-
tion and debug. A good silkscreen indicates the shape of the compo- nent, the orientation and any neces- sary labeling. Labels should be legi- ble and not covered by other compo-
nents. The end user must be able to identify any connectors, switches, or indicators for debug or configuration. The silkscreen is used during the lay- out portion to determine the compo- nents’ physical placements as well as to prevent any component conflicts during assembly.
Footprint Accuracy Footprint accuracy is the single
most destructive mistake that a lay- out designer or engineer can make. Unfortunately, component specs are not standardized, footprint drawings are not always to scale and some specs are drawn from a bottom view
Pin pitch is a
cumulative error and rarely is a problem with low pin-count items.
as opposed to a top view. In addition, many components do not fit on the manufacturer’s recommended foot- print and the controlling dimensions are not always clear. But using a CAD tool built to verify footprints can save schedules and rework because it builds a model to a compo- nent’s actual dimensions, as well as overlaying the model of the physical component over the footprint gener- ated in the design tool. Using the lat- est spec ensures that the component is the latest revision. The DFA check covers pin pitch, row pitch, pin type,
Continued on next page
Stay Connected
Page 1 |
Page 2 |
Page 3 |
Page 4 |
Page 5 |
Page 6 |
Page 7 |
Page 8 |
Page 9 |
Page 10 |
Page 11 |
Page 12 |
Page 13 |
Page 14 |
Page 15 |
Page 16 |
Page 17 |
Page 18 |
Page 19 |
Page 20 |
Page 21 |
Page 22 |
Page 23 |
Page 24 |
Page 25 |
Page 26 |
Page 27 |
Page 28 |
Page 29 |
Page 30 |
Page 31 |
Page 32 |
Page 33 |
Page 34 |
Page 35 |
Page 36 |
Page 37 |
Page 38 |
Page 39 |
Page 40 |
Page 41 |
Page 42 |
Page 43 |
Page 44 |
Page 45 |
Page 46 |
Page 47 |
Page 48 |
Page 49 |
Page 50 |
Page 51 |
Page 52 |
Page 53 |
Page 54 |
Page 55 |
Page 56 |
Page 57 |
Page 58 |
Page 59 |
Page 60 |
Page 61 |
Page 62 |
Page 63 |
Page 64 |
Page 65 |
Page 66 |
Page 67 |
Page 68 |
Page 69 |
Page 70 |
Page 71 |
Page 72 |
Page 73 |
Page 74 |
Page 75 |
Page 76 |
Page 77 |
Page 78 |
Page 79 |
Page 80 |
Page 81 |
Page 82 |
Page 83 |
Page 84 |
Page 85 |
Page 86 |
Page 87 |
Page 88 |
Page 89 |
Page 90 |
Page 91 |
Page 92 |
Page 93 |
Page 94 |
Page 95 |
Page 96 |
Page 97 |
Page 98 |
Page 99 |
Page 100 |
Page 101 |
Page 102 |
Page 103 |
Page 104