Mike see the reply below regarding the baking issue. The reference
material is from IPC-PE-740A troubleshooting guide which we are nearing
completion.
Regards
Dave
On Mon, 9 Oct 1995, Mike Cussen wrote:
>
> I ran into a delamination problem, with an individual supplier on two
different multilayer boards (FR4) after the IR process in our SMT assembly. >
> My question is, is it nessesary to bake these blank boards prior to the SMT
> process and if so, what are some guide lines I may use.
> 1. Tempurature 230 degress F. for 2 hours
> 2. Stacking Requirements in the oven?
> a. Separation between layers.
>
>
> Delamination & Blistering
> IPC-A-600D
>
> Although it may occasionally occur because of a sever process
> breakdown, delamination and blistering occurs as a result of an
> inherent weakness of the material. Either condition represents a
> breakdown in the cohesive strength of the composite laminate and
> the internal openings that develop can mechanically weaken in the
> final product and also provide undesireable conductive paths
> between circuit elements.
>
> Delamination or blistering may occur in clad laminate materials
> either between plies of laminate, or between the laminate and the
> foil cladding.
> Any occurrence, either in the as-received condition or after
> routine thermal stress testing at receiving inspection is
> evidence of a major flaw, and the material should be rejected,
> since the defect will almost always propagate during the normal
> stresses of manufacturing.
>
> Causes of Delamination
> "Quality Assessment of Printed Circuit Boards"
>
> The reason for delamination is epoxy starvation in the glass
> cloth layers or an incomplete curing of the base material. It has
> been reported that some manufacturers of laminates have reduced
> the lamination press cycle a little in order to increase
> production.
> In multilayer boards, visible delamination may be due to poor
> lamination of layers or insufficient bonding of prepregs to the
> inner layer copper circuits. Here, the PCB manufacturer is in a
> way also the "manufacturer" of the base material.
>
> Delamination may develop, however, in a board after assembly and
> soldering. There are several possible reasons for this:
> 1. Asymmetrical buildup of multilayer boards.
> 2. Faulty soldering techniques, such as skipping the baking which
> removes retained moisture from the board.
> (A 1 to 2 hour baking at 230 degress Fahrenheit immediately
> before assembly and soldering is highly recommended.
> 3. Lack of preheating as the first step of mass soldering.
>
>
> Delamination & Blistering
> IPC-A-600D
>
> Although it may occasionally occur because of a sever process
> breakdown, delamination and blistering occurs as a result of an
> inherent weakness of the material. Either condition represents a
> breakdown in the cohesive strength of the composite laminate and
> the internal openings that develop can mechanically weaken in the
> final product and also provide undesireable conductive paths
> between circuit elements.
>
> Delamination or blistering may occur in clad laminate materials
> either between plies of laminate, or between the laminate and the
> foil cladding.
> Any occurrence, either in the as-received condition or after
> routine thermal stress testing at receiving inspection is
> evidence of a major flaw, and the material should be rejected,
> since the defect will almost always propagate during the normal
> stresses of manufacturing.
>
> Causes of Delamination
> "Quality Assessment of Printed Circuit Boards"
>
> The reason for delamination is epoxy starvation in the glass
> cloth layers or an incomplete curing of the base material. It has
> been reported that some manufacturers of laminates have reduced
> the lamination press cycle a little in order to increase
> production.
> In multilayer boards, visible delamination may be due to poor
> lamination of layers or insufficient bonding of prepregs to the
> inner layer copper circuits. Here, the PCB manufacturer is in a
> way also the "manufacturer" of the base material.
>
> Delamination may develop, however, in a board after assembly and
> soldering. There are several possible reasons for this:
> 1. Asymmetrical buildup of multilayer boards.
> 2. Faulty soldering techniques, such as skipping the baking which
> removes retained moisture from the board.
> (A 1 to 2 hour baking at 230 degress Fahrenheit immediately
> before assembly and soldering is highly recommended.
> 3. Lack of preheating as the first step of mass soldering.
>
>
Reply follows
>From [log in to unmask] Tue Oct 10 08:02:31 1995
Date: Tue, 9 May 1995 13:01:41 -0500 (CDT)
From: Jon Holmen <[log in to unmask]>
To: [log in to unmask]
Cc: [log in to unmask]
Subject: Re: BAKE TIME (fwd)
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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