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| Conformal Selective Wave Solder Carriers - Design Rules | |
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The design of an exclusion fixture is "non trivial". As well as checking the process suitability, calculations need
to be done to ensure | |
These calculations are only aids and we have found that sucessful designs evolve with experience.
Our first exclusion fixture was over 13,000 jobs ago.
The viability of this process route crucially
relies upon being able to estimate the number and distribution of PTH
components
which CAN'T be processed (with and without glue dotting) - These will
have to be miniwaved or hand soldered.
which are likely to be poorly soldered - These will require further inspection
and manual touch up.
This estimation may be done in three ways
If a PCB is available (preferably populated)
- our sales engineers can rapidly evaluate your board.
If PCB design data is available we will process, analyse and remotely
assess it.
You can do it using the rules presented below - our customers quickly
find that the above two methods are easiest.
Pin Land to SMT pad clearance evaluation
The two figures below each show part of a CSWSC in plan and section views.
The right hand figure shows that more clearance
is required when the connector orientation
is perpendicular to the wave.
PTH
Components Located Parallel to direction through wave
The clearance required between
the pin land and SMT pad can be made quite
small, as the solder does not have to flow "under" the component
pockets.
A larger separation is required due to the solder having to flow "under"
component pockets.
Working your way around the underside
of the PCB, identify which components
are parallel and perpendicular to the wave and assess the solderability
of each
PTH connector by comparing the actual separation against the graph right.
Ideally you want to be above the line in all cases.
PCB Design Implications - for
Board Designers - or respin
We are often called upon by our customers to help with identifying design
respin opportunities.
We will identify problem areas within a board and suggest appropriate movements of components. (Ideally before the PCB is fabricated)
However for board designers reading this,
can you remember another four "rules" (to compete with the hundred
other rules you have to have floating
around in your head).
Keep large (height) SMT components away from PTH areas.
Leave the leading and trailing areas around PTH components as clear as
possible.
DON'T put any SMT components within 3mm (0.12") of any PTH components.
DON'T put all PTH components in line along one edge of a board - leave
some space to allow us to support the masking in the centre of the board.
