IPC-PE-740 "Troubleshooting for Printed Board Manufacture and Assembly" has words to the effect of �
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
220.127.116.11---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.
18.104.22.168---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.
22.214.171.124---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.
126.96.36.199---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:
* 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.