Performance Criteria

The critical performance criteria for electroless copper processes vary somewhat, depending on the application. Two primary criteria that relate to all applications are coverage and adhesion. Other criteria have become increasingly important over the past decade or so, especially for PTH.

Coverage relates to the uniform and complete deposition of copper over the substrate. Coverage defects may be referred to as voiding (PWBs) or skip plating (POP). Gross coverage defects that are easily discerned by the naked eye are typically caused by an improperly prepared surface or major abnormality in the overall plating process parameters. Pinpoint defects, which may also be detectable to the eye, have long been a recognized problem in the POP industry, due to the importance of the appearance of the final product. Since about the late 1970s, microscopy has been widely used to detect minute defects in PTH, due to increasing demands on circuit integrity. Typically a panel is sliced so that half the plated hole is visible for microscopic examination. Rather than incident light, backlighting is often used (Ref 64), which provides a very sensitive measure of assessing coverage. Figure 7 demonstrates levels of coverage that can be discriminated. (Both these through-holes would appear totally covered when viewed using incident light.) Some debate has taken place regarding whether "total" backlight coverage is necessary, or even advisable. The critical importance of deposit adhesion, for instance, requires that any mechanism employed to promote coverage, such as use of strong catalyst promoters, be carefully controlled so as not to introduce weak boundary layers in the plated composite.

Fig. 7 Backlight test cuopons. (a) Showing pinpoints voids. (b) Showing total electroless copper coverage

Adhesion of the copper deposit to the substrate is typically controlled by proper surface preparation. Typically the first two or three steps in the plating process prepare the surface in a gross manner. The subsequent steps of conditioning, microetch (for PWBs), catalyzation, and acceleration ensure that a complete, yet minimal, catalyst film is present, so as to maximize adhesion. Adhesion requirements vary, depending on the application. For POP the adhesion requirement is ~0.7 kN/m (4 lb/in.) peel strength, as measured by the Jacquet test (Ref 65), with good integrity to be maintained under a variety of environmental test conditions. The important factor is maintenance of a durable, cosmetic film over the lifetime of the part. The primary failure mode is blistering of the deposit. For conventional PWBs, peel strength per se is not usually measured. Adhesion is typically ascertained by cross-sectioning the board after electroplating, optionally subjecting the sample to high thermal stress that simulates or exceeds the expected stress to be induced during assembly, and then mounting and polishing prior to microscopic examination for failure between the electroless film and any of the other surfaces. Adhesion failure to the epoxy surface is commonly referred to as hole wallpullaway; adhesion failure to a copper inner layer is referred to as an interconnect defect (ICD). Detection and elimination of ICDs by improved electroless copper plating processes has led to significant improvement in PWB reliability over the past several years. Current industry standard electroless copper processes stand up to significantly more severe test conditions than those defined in the industry specification (Ref 66) (Fig. 8). In EMI shielding, the standard adhesion test involves scratching a cross-hatch pattern into the metallized film, then applying a tape test under prescribed conditions (Ref 67). This test is often used following a variety of environmental storage conditions.

Fig. 8 Solder shock test coupons. (a) Showing interconnect defect. (b) Showing strong interconnect bond after multiple solder shock exposure. Courtesy of Shipley Co.

Deposit quality has become increasingly important over the past ten years. Controlled grain size, low intrinsic stress, and low electrical resistivity are some of the most important qualities sought. A method of PWB reliability testing using elevated-temperature fluid bed media has been employed to rapidly assess the long-term reliability of the copper deposit in through-holes (Ref 16).

Control of the copper bath is now routinely achieved, in contrast with the unstable and unpredictable formulations of the 1960s and 1970s. Plating rate, rate of depletion, deposit characteristics, and so on are all maintained consistently throughout the lifetime of the bath, allowing statistical control procedures to be readily implemented.

Solderability. The effect of the through-hole copper plating bath and process on solderability has been the subject of several studies (Ref 68), because outgassing or "blow holes" (Fig. 9) occurring during the assembly process can significantly affect PWB end-product reliability. It has been shown that outgassing is dramatically reduced by a permanganate-based surface preparation treatment used for desmearing multilayer boards in combination with a fine-grain, low-stressed copper deposit (Ref 24).

Fig. 9 (a) Solder blow hole induced during solder shock testing. (b) Through-hole showing good solder fill after solder shock testing
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