The following is from section 1.5 of the latest draft of the IPC-PE-740
"Troubleshooting for Printed Board Manufacture and Assembly" which we are
hoping to publish 4Q 95.
1.5---BAKING
Laminate materials are baked several times during the manufacture of a printed wiring
board. There are four major reasons for baking:
1. To insure proper cure of the laminate resin.
2. To relieve stresses that may impact the dimensional stability of the
laminate.
3. To remove volatiles, e.g. moisture.
4. To cure any organic coatings that may be applied to the laminate (e.g.
thermal cure solder mask).
1.5.1---General Problems Associated With Baking
1.5.1.1---Cross Contamination.--- Certain baking operations volatize (drive off)
materials which may contaminate subsequent work process through the oven. Consider
separate ovens for these functions improved venting, or more frequent cleaning.
1.5.1.2---Excessive Baking (Over-Temperature Or Over-Time).--- In addition to
excessive oxidation of metallic cladding, excessive baking can change mechanical and/or
chemical properties of some laminates. Verification of actual oven temperatures (vs. set
points) and a mechanism for enforcing removal at the specified time are required.
1.5.1.3---Improper Loading.--- Stacking can result in uneven time/temperature
profiles with variation in results. Stacking also has the potential for trapping
contaminants between laminates at high temperatures, possibly degrading the metallic
cladding's surface. Racks or frames improperly designed or maintained can lead to
surface damage.
1.5.1.4---Hold Time After Bake.--- Uncontrolled dwell times after bake can defeat
the original purpose of the bake, especially moisture removal.
1.5.2---Resin Curing.--- Fully curing the laminate resin is important to increase the
laminate resistance to moisture absorption, to reduce drilling smear, and possibly to help
reduce the incidence of copper cracking by increasing the dimensional stability of the
finished laminate. Generally, full cure of the laminate is done by the vendor prior to
shipment to printed board fabricators. Refer to laminate vendors recommendations for
correct lamination and cure parameters.
A common measurement for completeness of cure is glass transition temperature (Tg) of
the laminate resin. The Tg is an indicator of the laminate thermal performance
capability and will vary accordingly to the resin system used. To achieve the Tg specified
by the laminate vendor, the resin must be fully cured. Full cure is accomplished by
heating the resin above the Tg and holding it here for a sufficient time to achieve
maximum cross linking of the resin system. It is recommended that the PWB
manufacturer check with his laminate vendor for the recommended time and
temperature (to avoid thermally damaging the laminate or excessive cure). Any bake
above the resin Tg should be done under pressure/weight.
Two IPC test methods are available to check laminate for its state of cure (evaluating its
Tg). One is Thermal Mechanical Analysis of TMA method, IPC-TM-650 Method 2.4.23,
the second is the Differential Scanning Calorimetry or DSC Method IPC-TM-650
Method 2 4 25. Because the two methods typically yield different values, the method
used should be consistent with that used by the supplier. It is also wise to specify the
test method to the supplier.
In order to determine if additional cure is needed, a single sample can be tested two
successive times. During the first test, the sample's cure is advanced by the test which
essentially acts as a bake (if undercured). If the second test differs significantly, baking
is advised. If the two test values are nearly identical, further baking should not be
required.
1.5.3---Stress Relief.--- Stresses are inherent in the lamination process and are the
result of the naturally occurring mismatch between the various constituents of the
laminate. An example of this is the different coefficients of thermal expansion of the
copper and resin system. Another example is the remaining stress from the original yarn
fabrication where glass strands are twisted and plied.
Stress relief assists in improving dimensional stability of the laminate by relaxing the
stresses prior to further processing. This is especially important for MLB core material.
As with increasing cure of the resin, stress relieving requires that the laminate be
elevated to a temperature above the glass transition temperature of the resin. Stress
relief may well be accomplished at the same time as increased cure is being effected, as
long as a slow cool down is used. In order to prevent bow and twist, the bake and cool
down should be done under low pressure or under a uniform weight.
Typical bake times and temperatures for the various laminate resins are shown below:
Resin Type Tg* Bake Temperature Time
---------------------|-----------|-------------------------|----------------
Difunctional Expoxy 130oC 160oC(320oC) 2-4 hours
Multifunctional Epoxy 145oC 175oC(350oC) 2-4 hours
Tetrafunctional Epoxy 150oC(350oC) 2-4 hours
BT Epoxy 180oC 190oC(375oC) 2-4 hours
Cyanate Ester 245oC 220oC(425oC) 4 hours
Polyimide 260oC 220oC(425oC) 4 hours
* Tg data obtained using DSC Method (IPC-TM-650 method 2.4.25)
Bake times are "at temperature," that is, after the material has reached the bake
temperature.
1.5.4---Moisture Removal.--- Resin systems used for PWB laminates vary in their
tendency to absorb moisture. Some resin systems (such as PTFE) are virtually
impervious, others can be rather hygroscopic. Residual moisture in the laminate has
been shown to cause a variety of deleterious effects from measles to blow holes to
excessive resin flow during lamination. Because of this, moisture removal baking is
advantageous and may be employed at the following stages:
* After oxide coating of innerlayers
* After hole preparation
* Before fusing
* Before solder coating/solder leveling
* Before any soldering operation, including wave solder and rework
The bake for moisture and volatile removal is typically done at 100=125oC for 2-4 hours.
Panels should be separated to permit air circulation: stacks of panels will not heat
properly and the moisture will not be able to escape. The user must note that the most
practical temperature and time conditions must be determined for each facility and each
printed board assembly.
When assemblies must be set aside (e.g. secondary component mounting of soldering
operations), the assembly should be stored in a desiccator cabinet at room temperature
and 40% maximum relative humidity (Desiccants may be baked out and reused.)
1.5.5---Organic Coating Cure.--- Follow vendor's recommendations for each curing
or drying process.
****************************************************
Jon Holmen
Technical Project Manager
IPC
7380 N. Lincoln Ave
Lincolnwood IL 60646
Phone (708) 677-2850
Fax (708) 677-9570
e-mail [log in to unmask]
*****************************************************
> Date: Tue, 09 May 95 11:04:00 CST
> From:[log in to unmask]
> To: [log in to unmask]
> Subject: BAKE TIME
>
>
> DEAR FELLOWS :
>
> WE ALL KNOW, BAKING PC BOARDS IS A COMMON
> PRACTICE FOR THE ELECTRONIC INDUSTRIES. THIS
> IS TO ELIMINATE ANY POSSIBILITY OF INTERNAL
> HUMIDITY. I WILL APPRECIATE IF SOMEONE CAN
> PROVIDE ME WITH INFORMATION ON WHAT IS THE
> REQUIRED TIME TO BAKE A PC BOARD? (IF THERE'S
> ONE) OR HOW CAN I EVALUATE THE AMOUNT OF TIME
> REQUIRED TO BAKE PC BOARDS?
>
> ROBERT NEGRON
> DSC OF P.R.
> (809) 882-8282
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