The Circuit Designer's Companion - diagramas.diagram...

62 The Circuit Designer’s Companionresult, strains set up under thermal cycling can crack the component itself or the trackto which it is soldered.You should not therefore use the larger ceramic or LCC components directly onepoxy fibreglass board. Leaded SM components such as small outline or flat-pack ICsdo not suffer from this problem because there is a section of compliant lead between thesoldered joint and the package. Plastic leaded chip carriers (PLCCs) with J-leadconstruction are useable for the same reason, and small chip ceramic components arealso useable, because of their small size.Cleaning and testingCleaning an SM board is trickier than for a through hole assembly because there is lessof a gap underneath the packages. Flux contamination can get into the gaps but it isharder to flush out with conventional cleaning processes. There is considerable effortbeing put into developing solder fluxes that do not need to be cleaned off afterwards.Testability is an important consideration. It is bad practice to position test probesdirectly over component leadouts. Apart from the risk of component damage, thepressure of the probe could cause a faulty joint to appear sound. All test nodes shouldbe brought out to separate test pads which have no component connections to them, andwhich should be on the opposite side of the board to the components. There is an extraboard space overhead for these pads, but they need be no more than 1mm in diameter.Testing a double-sided densely packed SM assembly is a nightmare (see the section onJTAG/boundary scan testing, section 9.3.3 on page 302).2.3.2 Package placementThere is a mix of electrical and mechanical factors to consider when placingcomponents and ICs. Normally the foremost is to ensure short, direct tracks betweencomponents and it is always worth interactively optimising the placement and trackrouting to achieve this. You may also face thermal constraints, for instance precisioncomponents should not be next to ones which dissipate power, or you may have toworry about heat removal.Over and above individual requirements there is also the general requirement ofproducibility. There should be continuous feedback between you as circuit designer andthe production and service departments to make sure that products are easy, andtherefore cheap, to produce and repair. Component and package placement rules shouldevolve with the capabilities of the production department. Some examples ofproducibility requirements follow:• pick-and-place and auto-insertion machines work best when componentsand packages are all facing the same way and are positioned on a welldefinedgrid• small tubular components (resistors, capacitors and diodes) should conformto a single lead pitch to minimise the required tooling heads. It doesn’tmatter what it is (0.4" and 0.5" are popular) as long as it’s constant• inspection is easier if all ICs are placed in the same orientation, i.e. witheach pin 1 facing the same corner of the board, and similarly all polarisedcomponents are facing the same way• spacing between components should take into account the need to get testprobes and auto-insertion guides around each component• spacing of components from the board edges depends on handling and wave