Simply tell your design guys to design single sided boards, problem solved!!
8^)
Royce
*************************************************************************
Royce Taylor
Process Engineering/Production
Suntron Corporation NEO
104 Glenn Street
Lawrence, MA 01843
(978)747-2061 voice
(978)747-2010 fax
[log in to unmask]
-----Original Message-----
From: TechNet [mailto:[log in to unmask]]On Behalf Of Hal Winslow
Sent: Friday, August 06, 2004 2:38 PM
To: [log in to unmask]
Subject: Re: [TN] Double sided boards and reflow
Joe,
Do yourself a favor and try and look back through the archives.
BGAs (well, plastic ones anyway) that go through a secondary reflow actually
become MORE reliable, as the solder joints elongate, and can flex more to
accommodate thermal stresses. Werner has stated this in many replies over
the years.
(Clip the following before you show the designers this email.)
Tell the designers to be designers and let the manufacturing engineers be
manufacturing engineers. This kind of thing drives me absolutely INSANE.
There isn't any chance you are going to go splashing around in their pond
telling them to use a different type of diode, or an FCT instead of an ACT
logic device, yet they think they can just reach into the complex
manufacturing process and completely change a solder alloy with no
ramifications. Because they think there "must" be a problem with a process
which is ubiquitous.
Sorry for the rant. The week must be over, time to go home.
Hal
-----Original Message-----
From: TechNet [mailto:[log in to unmask]]On Behalf Of Macko, Joe @ IEC
Sent: Friday, August 06, 2004 2:17 PM
To: [log in to unmask]
Subject: Re: [TN] Double sided boards and reflow
All,
Thanks for the input. My feelings exactly but I had to ask (-- action
item). Their concern is not about the parts falling off but about the
reflow of the solder joints again on the 1st side. Especially, any BGAs if
populated.
joe
-----Original Message-----
From: Tom Gervascio [mailto:[log in to unmask]]
Sent: Friday, August 06, 2004 10:59 AM
To: [log in to unmask]
Subject: Re: [TN] Double sided boards and reflow
Both sides will go into reflow but the surface tension of solder holds
the bottomside SMT parts in place. Threads from the TechNet address the
component weight versus the surface area of solder (if the ration of the
two is less than 30 the part will probably remain in place). Literally
millions of boards have been build with a double sided process over
decades.
Using different alloy will drive you to alloys where you might have to
prove it performs as well mechanically, electrically as eutectic
tin/lead. IBM did a lot of work with Sn-Bi which melts at 138 C -
however if lead is present you might be a ternary alloy with a very low
melting point and provide for reliability problems.
My $0.02 worth
Tom Gervascio
Senior Process Engineer
Sparton Electronics
(352) 540-4040
>>> [log in to unmask] 08/06/04 12:33PM >>>
Good Morning,
Many of our board designs are double sided with components on each
side.
When I explained the build process, a few of the design guys expressed
concern about the 1st side seeing another reflow when the board goes
through
again for the 2nd side and asked me to look into the practice of using
solder paste with different melting temperatures for the 1 st side
reflowed
vs. 2nd side reflowed. For example, use a solder paste/alloy with a
higher
melting temperature for the 1st side reflowed and a lower melting
temperature for the 2nd side so the 1st side reflowed does not reflow
again
(or minimize the impact on the 1st side) during 2nd side reflow. Some
of
the guys remember seeing CMs changing solder paste for the 2nd side
assembly.
Would anyone like to share their experience with this practice or
comment on
its potential value. thanks again.
joe
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