I'd like to add a little to what John has said. Copper foil actually has a
slightly larger CTE (16.6 ppm/C). When you look at a raw piece of copper-clad
laminate, the copper foil is in tension and the glass-epoxy is in compression
due to the mismatch. When you evaluate it for dimensional stability and look
at it right after etch and before any bake, you can see that the laminate has
actually grown a little. The bake cycle is where the shrinking begins. You
should also see that the thicker copper weight is, the greater the change is
also.
Glenn Pelkey
Quality/Reliability Engineer
Maxtek Components Corp.
John Nieznanski <[log in to unmask]> Wrote:
|
| Wilcox Tom wrote:
| >
| > We are experiencing a high degree of warpage on several
| different
| > multi-layer PWBs. The PWBs are typically 8 layer, .063
| thick GFN material
| > approximate size of 8"x10". Warpage is on the order of
| > .20" across a 13"
| > diagonal. The PWBs have 1" wide break-aways on two
| opposite outside edges.
| > Break-aways are GFN with no copper layers. The copper
| layers in the PWB are
| > 1/2 or 1 oz and are fairly well balanced around the
| mid-plane of the board.
| > Numerous interstitial vias are used and all are located
| in the bottom half
| > layers of the PWBs.
| >
| > Not all PWBs we receive are warped. Recent runs show a
| 10% reject rate for
| > warpage. It appears that some boards also warp after we
| receive them (i.e.
| > Manufacturer says they past his inspection.) Some boards
| warp after reflow.
| >
| > We are looking for any possible causes of the warpage.
| There is some thought
| > that the unbalanced interstitial vias may be causing the
| warpage. Any
| > suggestions as to cause of corrective action would be
| appreciated.
| >
|
| While attending the PWB Design Conference in Santa Clara
| in March, this
| subject came up during the High Speed Design program
| during a discussion
| of the advantages and disadvantages of various board
| stackups. One of
| the instructors (Lee Ritchey, 3 Com) indicated that in his
| experiences
| in tracking board warpage problems, it always turned out
| to be a
| supplier and/or vendor quality control problem and not a
| design problem
| due to "asymmetrical stackup".
|
| If we look at this in more detail, there seem to be many
| "gotchas" in
| the PWB manufacturing process which need to be effectively
| addressed to
| ensure your PWB isn't warped.
|
| Due to the nature of their construction and the processes
| used in
| manufacture (heat and pressure), laminates exhibit a
| different expansion
| factor in each of the three dimensions. Typical CTE values
| (PPM / deg C)
| listed for an FR-4 epoxy fiber-glass laminate are: X
| (length) 12, Y
| (width) 16, Z (thickness) 60. The linear expansion of a
| material is
| calculated as the board dimension, times the CTE, times
| the temperature
| change. Laminated materials undergo size changes during
| the board
| manufacturing processes, some more than others. Excessive
| size change
| contributes to board warpage and layer-to-layer
| registration problems.
| The laminate size change is caused by the difference in
| CTE between the
| base material and copper. In the laminate manufacturing
| process, the
| materials are subjected to high temperatures and pressure
| in the
| laminating press. As the copper clad laminates cool down,
| the resin
| wants to shrink. However, the copper foil laminated to the
| surface has a
| lower CTE and does not want to shrink as much. The result
| is a locked
| in stress between the base material and the copper foil.
| When the copper
| foil is etched away, the residual stresses are relieved
| and the laminate
| material shrinks.
|
| To make up the various dielectric thicknesses required,
| multiple layers
| of standard thickness prepreg have to be built up into
| copper-clad
| laminates, with the right materials, in the right
| sequence, with the
| right recipe. Without knowing any of the details of what
| your vendor is
| doing, several things to look at would be:
|
| Differential CTE mismatch in X and y dimensions, CTE
| mismatch due to
| misorientation of prepreg materials, different and/or
| variable
| prepreg/laminate suppliers, different moisture
| absorptions, uncontrolled
| prepreg/laminate storage environment (temperature and
| relative
| humidity), prepreg/laminate shelf life in uncontrolled
| environment,
| incompatible prepreg formulations, etc.
|
| Due to the wide range of pathologic behavior you are
| expereincing, my
| guess is that you are seeing some combination of the above
| factors.
|
| Hope this helps.
|
|
| --
|
| ==========================================================
| ==
| John Nieznanski Eastman Kodak Co.
|
| [log in to unmask] 2/4/EP MC35313
|
| PH: 716-726-2520 901 Elmgrove Rd.
|
| FX: 716-726-0745 Rochester, NY
| 14653-5313
| ==========================================================
| ==
|
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