|
 |
|
  |
|
|
Solder Reflow "Sweat" Bonding
Introduction
The following is an overview of the purpose and procedure for attaching a
Teflon® printed circuit board to a metal heatsink, carrier or pallet. For the
purpose of this report heatsink, carrier and pallet may be used
interchangeably.
There are three advantages to sweating or bonding a printed circuit board to a
carrier. One, to provide stiffness/rigidity to the circuit board making general
handling, solder stenciling, pick and place or hand placement of components
easier as well as to prevent warping or distortion of the finished assembly
during wave, IR or convection soldering/reflow operations. Two, to maintain
electrical conductivity and thus a continuous ground between the circuit board
ground plane and the carrier. Three, to provide thermal conductivity between
the Teflon® printed circuit board and a heatsink to allow dissipation of heat
from the circuit board and high power components such as power transistors.
Definitions
Sweating is the attachment or bonding of two substrates using solder.
Sweating may be performed using electroplated solder or by the addition of a
solder paste having various compositions depending on the assembly process
and design requirements.
Solder Plated Substrates
A tin/lead (solder) plated circuit board and either a solderable carrier (e.g.
brass or copper) or a solder plated carrier* may be sweated directly together
using flux to provide surface activation and to prevent oxidation during
soldering.
*Tin/lead may be directly plated on brass and copper. Aluminum requires
zincating prior to plating electroless nickel, which can then be followed by
electroplated copper, tin/lead, or gold to provide solderability.
Substrates using Solder Paste
Solder paste consists of finely milled particles of solder mixed with flux. Using
solder paste is a method of sweating two solderable but non solder coated
surfaces and or obtaining a higher melt point for the solder joint using a
different ratio of tin/lead from the eutectic point (63% tin/37% lead having
one of the lowest melt points).
Procedure
1. Surface Preparation
Gently abrade the two surfaces to be sweat bonded using an abrasive pad
such as Scotch-Brite 600¹. This will remove any oxidation from the solder
plating and will produce a clean and solderable surface on copper, brass and
gold (Caution: immersion gold plating may be removed if excessively
abraded).
2. Applying Flux or Solder Paste
Evenly coat both surfaces with a low volatile/no clean flux or with solder paste
containing no clean flux. This may be done with a fine bristle brush or solder
paste can be screened on using a mesh and fabric type recommended by the
solder paste manufacturer. Allow most of the solvents to evaporate before
assembling.
3. Fixturing
Alignment between the circuit board and carrier is usually achieved by
inserting pins into holes of the same size and location. Even pressure is
then applied using a spring loaded clamping device, multi- layer press or
a weighted object. Use only enough pressure to keep both substrates in
contact with each other during soldering and cool down operations.
4. Reflow/Soldering
With the pieces fixtured together apply heat to the package using a
preheated (See Graph 1) convection oven, or hot plate (when using spring
loaded or weighted fixtures) or place the package into a preheated multilayer
press. Allow the package to come to above the solder melt temperature then
remove it from the heat while under pressure (spring loaded or weighted
fixtures) and allow it to cool to below the melt temperature of the solder.
When using a multilayer press turn the platen heaters off and allow to cool
under pressure or move to separate cooling press (while hot) and reapply
pressure until cool. IR reflow is not recommended.
5. Cleaning
Remove the pieces from the fixture and clean any remaining flux residue
from the board edges, cutouts and from within the through holes using a
suitable solvent. Dry the assembly with a paper towel or soft cloth. Any
oxidation may be removed using the same method as described in step 1.
|
|
|
|
|
|
|
