Pradeep,
Those are some loaded questions, and the real answer (of course) is "it
depends", but I will attempt some answers for you..
1. If you follow J-STD-001, reference section 4.9.1 - Preheating.
Basically, you must preheat the boards for machine soldering. This is to
reduce the thermal shock to the PCB when it contacts molten solder, such as
in wave or selective soldering. An exception is automated SMT. We do not
pre-heat boards for this process, as the pre-heated board will cause the
solder paste to soften when it is screened on, and the reflow profile
hopefully alleviates the thermal shock by maintaining a proper ramp. And
while not required by J-STD-001, pre-heating the PCBs can also be very
beneficial for hand soldering operations, particularly on thicker backplanes
with copper ground planes.
Time and temperature of pre-baking can vary widely. You must consider the
materials and components involved. IF the PCBs are bare, you only need to
worry about the PCB materials. There are so many combinations and
variables, there is no industry standard / specification - at least that I
know of.
Also know that any baking will act to reduce the solderability of the PCB,
so you want to minimize the process.
2. Pre-baking between various stages would depend on the environment and
duration the assembly is subjected to between processes, and also any
intermediate processes. For example, if you are assembling in a very humid
environment, pre-baking can help drive out moisture absorbed by the
assembly. If you clean the PCBs between processes, and the board is not
dried sufficiently, there will also be absorbed moisture.
3. This is a general question, but a great place to start is IPC-A-610 for
requirements, and J-STD-001 / IPC-HDBK-001 for process recommendations.
Hope that helps.
Kevin Glidden
Manufacturing Engineer
Luminescent Systems Inc.
-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Pradeep
Sent: Friday, May 12, 2006 5:02 AM
To: [log in to unmask]
Subject: [TN] PREHEATING
Importance: High
Can somebody clarify on few queries wrt the preheating of the bare boards
befor assembly.
1. Is preheating a must and if so at what temperature and for what
duration should it be carried out. Is there any standards specifying this
2. If the assembly of the board is being carried out at different phases,
is preheating required before each stage
3. Can anybody help on the specifications on hand soldering of SMD
components
Pradeep
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