I recommend hand solder using wire and pot the beegeebees out of it using the existing lands as terminals. Forget about 1st pass through reflow, this is a modification.
----- Original Message -----
From: Bogert, Gerald L (Contractor)<mailto:[log in to unmask]>
To: [log in to unmask]<mailto:[log in to unmask]>
Sent: Thursday, March 31, 2011 6:28 AM
Subject: [TN] Soldering two SMT resistors to lands originally designed for only one resistor; is it technically OK?
March 31, 2011
Folks, we have an OEM who needs to incorporate a design change into an
existing design circuit card assembly (CCA) used in a Class 3 military
application that will see shock, vibration and high humidity
environment.
In the existing design, a chip resistor is soldered to land areas with
the following dimensions: Width is 0.080 inches length is 0.035 inches.
Spacing between lands is 0.040 (makes the overall length 0.110).
The OEM needs to remove the existing resistor and solder two new
resistors to the existing land areas. The new resistors are
M55342K06B4E02R (MIL-PRF-55342/6, 0.15W, 1%, 4.02K). From the military
specification, the overall part length is 0.080 inches; the width is
0.050 inches, and the termination width is 0.016 inches. The tolerance
on the 0.050 dimension is plus or minus 0.005 inches.
From the above information, I calculate that when the two resistors are
soldered onto the existing lands, the parts will overhang on both sides
of the lands by 0.015 inches (assumes the resistors are spaced 0.010
inches apart to account for the 0.005 inch tolerance on the 0.050
dimension). From IPC-A-610E, paragraph 8.3.2.1,and Table 8-2, the
maximum allowable overhang for Class 3 product would be 0.0125 inches,
and for Class 2, 0.025 inches.
This means that side overhang would not meet Class 3 for Dimension A,
but would meet Class 2 requirements.
Is this design change technically acceptable for Class 3? Given the
above information, is there sufficient solder joint formation (assuming
perfect soldering process) to provide the requisite level of
reliability? What is the relationship of side joint overhang to
reliability? What data exists to support the A Dimension criteria in
Table 8-2 of IPC-A-610?
The OEM is considering obtaining additional solder joint volume by
soldering a jumper wire from the end terminations of each resistor to
the land area such that the jumper wire would act as an additional
soldered conductor. He is also considering whether the parts should be
glued on the sides for support.
It appears that it would be acceptable to just solder both resistors as
described above, without the need to add the jumper wire or glue, which
would mean we would accept the violation of Dimension A for Class 3.
Note also that there is no electrical clearance concern using this
design fix and there is no known other acceptable design fix; if this
fix is not technically acceptable the CCAs would need to be scrapped..
We considered stacking the resistors on top of each other but this was
rejected because; 1) not recommended by the military specification; 2)
cannot guarantee that the top surface of the terminations are solderable
- the mil spec exempts this surface from solderability test
verification; and 3) possible heat transfer issues.
Any advice folks could provide to the above would be appreciated.
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