Hi Marius,
It's my understanding that in the assembly re-flow machine they 'pre-soak'
the board with enough heat to bring the components and their pads just below
the plastic melting stage of solder. Then as the board travels through the
oven, there is a window of heat that is sharply higher in temp that reflows
the solder quickly as it passes thru that zone. Afterwards it is allowed to
cool under a controlled environment to where it exits the machine. The more
mass to heat, the longer the soak time required. This means that cost is
higher for boards requiring longer soak times to come up to temp due to the
energy and time required. The goal would be to balance the electrical and
thermal requirements of the part with the soldering requirements. Of course
these are difficult things to model. Many people don't discover that there
is a problem until the board is built. Copper is one of the best heat
spreaders and can conduct thermal energy very well.
You may find that you can use thermal relief pads where you thought it not
possible. I always add up the widths of the connecting spokes to come up
with an effective trace width for the connection to a plane. Copper
thickness can affect thermal performance as well.
I am not aware of any guides out there for this specific question... maybe
someone else knows of one...I would check with my manufacturing engineers at
the assembly house or in your own company if you do re-flow in house.
good luck.
- Bill Brooks
-----Original Message-----
From: Marius Geurts [mailto:[log in to unmask]]
Sent: Wednesday, December 20, 2000 6:15 AM
To: [log in to unmask]
Subject: [DC] Heat transfer From and To the SMD leads.
Hi all,
I have the following question/problem.
Today there are a lot of components with special requirements regarding
thermal connections to draw the heat away from the package. I.e.
- Power devices applications, in a SOT-143, SOT.223, TO-263, SO-8
package.
- BGA applications with thermal via's that are connected to full GND
plane without,
thermal relieves.
* Component specification.
To meet the component specification of these devices within in his
application, a specific land pattern and interconnection in the PCB
design is required, to provide the heat transfer.
Some component manufactures specified copper planes, wide tracks,
thermal via's etc.
to draw heat away from the package.(see attachment, example)
* DFM.
This effects also the heat transfer to the solder joint during the
reflow soldering process,
to create a good solders joint
The following DFM rules are used to provide this heat transfer during
reflow soldering.
- Via to pad distance, to prevent solder pasta floating away from the
solder pad.
- Thermal pads, connected to copper plane or width track.
- Track width restriction to the solder pad connection
- etc.
With this we have a "Design rule conflict" between DFM and of the
Component specification.
* Design Rules.
To make a good design that meets the DFM - and Component specification,
I need to know the design rules for wave and reflow soldering for this
type of SMD applications. Especially the design rules for the
interconnection of the solder pad to a copper plane or wide tracks. To
insure the Manufacturability of this package with a high solder yield in
volume production and a solder joint quality according IPC-A- 610 class
2.
Another issue is the, reflow solder temperature profile that's needed to
create a good solder joint connection to copper planes, wide tracks etc.
This must meet the component soldering heat resistance to prevent
component damages.
(ref. CECC 00802 std. Soldering methods).
* Questions.
- Who can help me with design rules for these applications, to insure a
high solder yield in volume production. And a solder joint quality
according IPC-A- 610 class 2 ?
- Are these described in a IPC standard or any other document?
Best Regards,
Marius Geurts
Manufacturing Quality Engineer Electronics
Research & Development
Oce Technologies B.V.
The Netherlands
IPC member
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