This article describes how a recent research project discovered the influence of component pitch and board layout on the placement accuracy of CSPs and flip chips.
Stencil printing of solder paste is the fastest and most cost-effective method for high-volume electronic circuit manufacturing. Unfortunately, this method is no longer suitable for components with pitches smaller than 300 microns. For these components, the adhesive flux process is a better approach; it can be implemented directly on fine-pitch component placement machines equipped with flux dipping units.
The four steps in the adhesive flux process are: picking up the component, dipping the component's solder balls into the flux, aligning the component, and placing the component on the substrate. Because this process allows handling components with solder ball pitches as small as 100 microns, flip-chip assembly with eutectic solder balls may be particularly interesting.
When flip chips with eutectic solder balls are placed in solder paste, their positions are typically corrected during reflow through the self-alignment of the molten solder. Similarly, the positions of most components with eutectic solder balls placed in flux are also corrected through self-alignment. A recent research project investigated the relationship between solder joint formation and placement accuracy for eutectic solder balls placed in flux.
Preparation for the study
Due to limitations in obtaining components and substrates, extremely fine-pitch components were avoided to simplify these trials. Two types of components were used: CSPs with a 750-micron pitch and flip chips with a 450-micron pitch. The results were extrapolated to components with pitches as small as 100 microns. Two different board layouts were used: copper-defined pads for CSP placement and solder mask (RS)-defined pads for flip chips (Figure 1).
An advanced component placement machine was also used, with a 3-sigma accuracy of 9 microns, equipped with a flux dipping unit. A no-clean adhesive flip-chip flux, suitable for dipping, was used.
Verification trials were completed by adding offsets across three different copper pad sizes and three different solder mask openings. Components were placed with increasing positive and negative offsets in the X direction. All given offsets were measured from the nominal position. Two different test plans were executed for the CSPs and flip chips. A total of 450 CSPs and 450 flip chips were placed.
CSP trials
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The flux layer thickness in the flux unit was 95 microns, equal to 50% of the CSP solder ball height. The CSPs were first dipped in the flux unit and then placed on the test boards with five negative and five positive offset steps. For each offset step, 15 components were placed on three different copper-defined pad layouts:
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. Copper pad = solder ball diameter
. Copper pad = 85% of solder ball diameter
. Copper pad = 70% of solder ball diameter
