Printed Circuit Design & Fab - May 2008 - (Page 35)

FIGURE 6. Surface mount pad with cratering. FIGURE 7. Lifted pad associated with cratering. Definitions of Material Degradations: Adhesive Delamination. This condition is described as a physical breakdown between two laminated layers. This type of delamination occurs between the B-stage and copper foils, the B- and C-stage layers, or epoxy glass fiber bundles. This type of delamination is an interfacial type failure where cracks occur between laminated layers, or along glass bundles (not individual glass fibers). This failure is expressed as a crack proceeding down epoxy-to-epoxy bonds, epoxy-to-copper interfaces or epoxy-to-glass bundles. In a cross section, the crack is long, mostly horizontal and ends in a thin point. A top-down view of the delamination between laminated layers or copper may show a bubble or blister. Adhesive delamination along glass bundles may appear to be square or have a cross or X shape corresponding the knuckles of the woven glass. Although most material degradation is a result of many influences, the general opinion is that adhesive delamination is predominantly a mechanical failure. Aggressive out-gassing of volatiles (moisture or solvents) may be the cause for this type of failure. This failure mode is accelerated by problems in copper surface treatments (oxide or conversion coatings) or surface contamination on traces and particularly on internal pads and planes. Adhesive delamination may result when the lamination process has been compromised, when B-stage materials have exceeded shelf life or been improperly stored, or where the press cycles preheat, pressures, cure times or MAY 2008 cooling rates are set incorrectly. Drilling may also play a factor in adhesive delamination. Cracking along glass fibers may provide a place for aqueous substances to accumulate prior to electroless copper deposition. These cisterns offer storage sites for liquids that vaporize during high thermal cycles. This failure mode may be expressed in multiple boards from the same lot. Isolated blisters, pink ring, measling, and large internal material separations are usually a reflection of this failure mode. FIGURE 2 shows adhesive delamination between the B-stage and C-stage interface and copper surfaces. Cohesive Degradation. This condition, commonly described as cohesive delamination in the electronics industry, may be a misnomer in that it implies lamination is involved in the failure mode. A cohesive failure occurs within the base material, but is not implicitly associated with a laminated interface. Cohesive failures may be described as a breakdown of material within the B- or C-stage layers of the dielectric. This failure mode appears to be a degradation precipitated within the epoxy. A chemical degradation is most likely the dominant influence in cohesive failure, though it is undoubtedly aggravated by mechanical stresses like out-gassing of volatiles (FIGURE 3). This failure occurs when the forces exceed the strength of the material. Failures occur when either the forces are increased, or the material loses strength. It appears that, in some instances of cohesive failure, the failure may occur over time at an isotherm, which suggests the failure may be precipitated by a loss YOU’RE YOU’RE www.efabpcb.com Easy to use instant on-line quoting If you have any questions or need assistance Call Toll Free 1-877-332-2722 or Contact – sales@efabpcb Prototypes thru Production Check out our low prices and save big $ Save more with our Proto-Power Pack Specials! Quality PCBs at competitive prices! Let us be your on-line PCB shop Lead free / RoHS finishes available ENIG – Immersion Silver – Immersion Tin – OSP – Lead Free Solder efabpcb is a UL-approved – ISO9001:2000 certified facility PRINTED CIRCUIT DESIGN & FAB 35 http://www.photoplot.com http://www.efabpcb.com

Table of Contents Feed for the Digital Edition of Printed Circuit Design & Fab - May 2008

Printed Circuit Design & Fab - May 2008 Contents Our Line Market Watch Around the World Happenings ROI EMC For the Real World PCB East Conference Brochure Positive Plating Don't Let your Signals Stub Their Toes Improve PCB Layout With Skill Utility Programs The Next Generation Design Tool Challenge Thermally Conductive Microwave Materials PCB Dielectric Degradation in Lead-Free Assembly Applications A Tale of Two Trade Shows Eliminating Board Defects Off the Shelf Marketplace Ad Index BGA Bulletin

Printed Circuit Design & Fab - May 2008

For optimal viewing of this digital publication, please enable JavaScript and then refresh the page. If you would like to try to load the digital publication without using Flash Player detection, please click here.