An additional note,

It's unlikely to heat a board to flatness... If a board is warped, =
bringing
it above Tg again and pressing it flat MAY make it straight. But at =
anytime
it meets with a high temperature heat cycle again (like during any =
reflow
process), the strain may relieve itself and the board is highly likely =
to
warp again.
Also note that inner laminate crosslinkage increases every time a board =
sees
a heat cycle, including baking. Increased crosslinkage, makes the board =
a
bit more brittle and slightly reduces life-expectancy of the board...
Polymer chemistry is so much fun..... Well maybe not.

Later all,

Alain Savard, B.Sc.
Chemical Process Analyst
CAE Electronics Ltd.
e-mail: [log in to unmask]

-----Original Message-----
From: Colin W.T. McVean [mailto:[log in to unmask]]
Sent: July 13, 2000 6:05 AM
To: [log in to unmask]
Subject: Re: [TN] Glass Transition Temperature (Tg) Questions


The process will stop once the material reaches the "Temperature of
Degradation", which in the case of standard FR4 is around 320oC. At =
this
point, the bonds irreversibly break down, or decompose.

----- Original Message -----
From: michael <[log in to unmask]>
To: <[log in to unmask]>
Sent: Thursday, July 13, 2000 3:15 AM
Subject: Re: [TN] Glass Transition Temperature (Tg) Questions


> Epoxies are thermosetting. Surely this process is not indefinitely
two-way? Or
> should I say: beyond what temperature is the process no longer =
two-way
(and what
> is the technical term for this temp.)?
>
> Michael Fennema
>
> -----=AD=EC=A9l=B6l=A5=F3-----
> =B1H=A5=F3=AA=CC: Mike Fenner <[log in to unmask]>
> =A6=AC=A5=F3=AA=CC: [log in to unmask] <[log in to unmask]>
> =A4=E9=B4=C1: Thursday, July 13, 2000 05:58
> =A5D=A6=AE: Re: [TN] Glass Transition Temperature (Tg) Questions
>
>
> |Just for completeness, thet Tg is two way:
> |When ever the polymer is at a higher temperature than the Tg, the
> |chemical bonds are relaxed and the material is relatively floppy. =
When
> |the polymer goes back below the Tg the chemical bonds tighten back =
up
> |and the material reverts to its hard state. The transition from hard
> |to soft (or soft to hard) is a repeatable process in either =
direction.
> |
> |Mike
> |
> |----- Original Message -----
> |From: "Alain Savard" <[log in to unmask]>
> |To: <[log in to unmask]>
> |Sent: Wednesday, July 12, 2000 5:35 PM
> |Subject: Re: [TN] Glass Transition Temperature (Tg) Questions
> |
> |
> |> My answers marked with 2 asterisks (**) below.
> |>
> |> Alain Savard, B.Sc.
> |> Chemical Process Analyst
> |> CAE Electronics Ltd.
> |> e-mail: [log in to unmask]
> |>
> |> -----Original Message-----
> |> From: Hiteshew, Michael [mailto:[log in to unmask]]
> |> Sent: July 12, 2000 12:06 PM
> |> To: [log in to unmask]
> |> Subject: [TN] Glass Transition Temperature (Tg) Questions
> |>
> |> Hi,
> |>         I've been reading up on laminate materials and have some
> |questions
> |> that I hope some-one can help me with:
> |>
> |> 1. What is the "glass transition" ? What is the glass =
transitioning
> |from and
> |> to? I'm guessing it's the plastic stage that occurs in a material
> |just
> |> before it melts.
> |>
> |> ** Temperature at which the laminate turns from a relatively hard
> |and
> |> potentially brittle state, to a vicious, semi-liquid stage. Epoxy
> |used in
> |> the boards is more likely to decompose or burn than it is of ever
> |melting.
> |>
> |> 2. What is gained by a higher Tg in tetrafunctional and
> |multifunctional
> |> epoxy-glass?
> |>
> |> ** Higher Tg means that you have a better resistance to heat from
> |any
> |> source. Your board is more likely to keep it's shape during heat
> |related
> |> fabrication process or during use if it has a higher Tg.
> |>
> |> 3. What is meant by the prefixes "tetra" and "multi" as they're
> |applied
> |> here? Is it a reference to the number on bonds in the chemical
> |structure of
> |> the epoxy or glass?
> |>
> |> ** It refers to the number of linkage sites of the epoxy =
prepolymer.
> |>
> |> Thanks in advance,
> |>
> |> Michael Hiteshew
> |> Lockheed Martin NE&SS-Baltimore
> |> [log in to unmask]
> |> (410) 682-1259
> |>
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