Hi, Bill,
BGA subjects are a regular feature on the TN forum, and I asked a similar
question to yours not very long ago. I have also just been through the
profiling process for an ENIG finished FR4 board with 2 PBGA's and 1 CICBGA
onboard, so maybe my experience can help you a bit.
When I tried to research generally into how much heat components could
withstand, the answers I got ranged from loud silence to generic.
Apparently this information is commercially sensitive, and everyone else is
left to assume (rightly or wrongly) that components can withstand soldering
temperatures, whatever they may be. However, I did find out that components
are typically designed to withstand between 230 and 235 deg C before the
internals come apart. For BGAs and similarly-styled components, the
manufacturers are increasing this temperature to about 250 deg C, though
this doesn't help if other components are cooked in the process.
I wound up being able to use normal 63/37 eutectic solder, with a
liquidous/solidous temp of 183 deg C. Peak temperature was set to 215 deg C
for 18 seconds, during which the central ball of the BGA's reached 207 deg
C. Otherwise, the profile followed that provided by the solder manufacturer
(Kester). Temperature would be a much more delicate balancing act if using
high melting point (221 deg C), as the peak temperatures are then above
normal safety limits and the duration has to be very much shorter to 'sear'
the outside but not cook the inside of the components. Personally, I
wouldn't like to have to use high melting point solder with BGA's - the
time lag for the temperature to catch up is too great for the other
components.
Whether using solder manufacturers' thermal profiles is sound practice or
not, could be debated, but there is no other practice that I'm aware of
that doesn't involve a lot of hard work for possibly little gain. If the
profiles were badly out, there would be a lot of complaint from Industry to
improve them, but on the whole, they are a good place to start. Thermal
profiling has to be carried out anyway to tune the profile to the assembly
being processed, as the solder manufcturers' profiles are only generic.
To start with, a board manufacturer has to know the components being used
on a board and what temperatures they can withstand. Although these
temperatures do not appear on data sheets, component manufacturers seem
willing to supply it on request, even though this is a tedious proess. We
found we had a transformer that had to be hand soldered because it can't
withstand CR temperatures without distorting. Knowing what the lowest
maximum withstandable temperature is, you can them proceed to tune the
thermal profile using thermocouples in appropriate places on the board (on
any known thermally sensitive devices, underneath BGA's, by any large
thermal masses, etc.)
I was told by one of the TN gurus that liquidous + 20 deg C is normal for
normal components, but for BGA's and similar, he recommended to use
liquidous + 35 deg C to ensure adequate heat reached the shadiest parts of
the BGA. In fact, I opted for about +30 deg C in the end, as the profile
from the thermocouples showed the BGA temperature in the middle of the
components had reached liquidous + 20 deg C at this setting.
There is no instant formula since there are too many variables in any given
design, and all if your sources have given you 'safe' temperatures. From
the above, I hope you can see that 230 deg C is approximately the max temp
allowed for the component, 220 deg is the preferred peak temp setting for
reflow soldering as it generally lets the board achieve soldering
temperature of around 210 deg without risking damage to the component.
Hope this helps
Pete Duncan
Bill Butman
<bbutman@YAHO To: [log in to unmask]
O.COM> cc: (bcc: DUNCAN Peter/Asst Prin Engr/ST Aero/ST Group)
Sent by: Subject: [TN] BGA Max temps & times?
TechNet
<[log in to unmask]
ORG>
06/12/01
02:58 AM
Please
respond to
"TechNet
E-Mail
Forum.";
Please
respond to
Bill Butman
Techneters, I need your help:
I'm curious about the critical temperature and time
values regarding BGAs. Do the Component Manufactures
have one set of limits, and do PCB Assembly Shops
have another set of limits. Companies like ours are
many times removed from the component manufacturers.
The industry default profile slopes are generally
built on solder paste characteristics. Is this a
sound practice?
As I travel and interface with other engineers I find
that generally the solder paste manufactures recommend
a maximum temperature of 230 degrees C. While the
reflow oven profilers max.-out at about 220 degrees C.
Then the BGA rework engineers tend to agree that 210
degrees C. is the maximum temperature. And as for
times, well most discussions have them varying between
45 seconds to 90 plus seconds.
Therefore:
1. Are there any general rules for the maximum
temperature that BGAs can tolerate,?
2. Are there any general rules for the maximum time
that BGAs can tolerate elevated temperatures?
I thank you for sharing your thoughts and wisdom.
thanx
Bill Butman
Circuit Technology Center
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Bill Butman
Circuit Technology Center
45 Research Drive
Haverhill, MA 01832
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