I have a customer who is transitioning from 100% thru hole boards to
designing boards with smt components.

We gave them our basic DFM guidelines and they have laid out 2 boards
following these guidelines.  When I saw the result of their first pass, I
thought of two things.  1) we need to review our guidelines and 2) I need
some advice from Technetters.

The product is mil-aero building to IPC Class 3 using mil spec components.

The assemblies are all laid out for a process flow of
1) reflow topside smt
2) epoxy bottomside smt
3) stuff thruhole
4) wave solder

I have 2 questions:

1)  They have placed smt resistors as large as 2512's and capacitors up to
1812 on the bottomside.  I recall reading about problems with wave
soldering large capacitors and having reliability problems.  Would this
also apply to resistors?  What about the 1812 caps?

2)  They have also place 1206 resistors between the pins of .300 dips on
the secondary side.  We had them orient the resistors parallel to the wave
but unfortunately the dips are also oriented parallel to the wave (I need
to check our DFM doc).  With this, they have positioned 6 1206's between
the leads of a dip16 component and place 3 dip components in a row with 3
1206s between them.  ASCII art to follow.

o - reps a dip lead

[] - reps a 1206
[]

<-----  conveyor direction

o  []  []  o  []
o  []  []  o  []
o          o
o  []  []  o  []    repeated 3 times in this direction
o  []  []  o  []
o          o
o  []  []  o  []
o  []  []  o  []

Is anyone building anything like this successfully?  Volumes are less than
100/mo.

Any input is appreciated.

Peter Wong
[log in to unmask]

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