CSP and flip chip assembly using viscous flux (1)
SMT Technical literature
This article describes a recent research project that found out how component spacing and board layout affect CSP and flip chip placement accuracy.
Stencil printing of solder paste is the fastest and most cost effective method for high-volume electronic circuit manufacturing. Unfortunately, this method is not very good for components with spacing less than 300 microns. For these components, the viscous flux process is a preferred method; it can be carried out directly on a closely spaced component placement machine equipped with a flux dip coating unit.
The four steps in the viscous flux process are: picking up the component, soaking the component solder ball to the flux, centering the component, and mounting the component on the substrate. Flip-chip assembly of eutectic solder balls may be interesting because the process allows for the processing of components with solder ball pitches as small as 100 microns.
When a flip chip with eutectic solder balls is mounted in a solder paste, its position is typically corrected by self-centering of the liquid solder during reflow. Similarly, most component placement of eutectic solder balls in the flux is also corrected by self-centering. A recent research project investigated the relationship between the formation of solder joints and the placement accuracy of eutectic solder balls mounted in flux.
Preparation for research
Due to the limitations in obtaining components and substrates, the use of extremely small pitch components has been avoided here to simplify these tests. Two components were used: a 750 micron pitch CSP and a 450 micron flip chip. The results inferred to components with pitches as small as 100 microns. Two different board layouts were used: a copper-delimited layout for CSP placement and solder mask (RS)-defined layout for flip chip (Figure 1).
An advanced component placement machine with a 9-micron accuracy of 3 sigma is also available, equipped with a flux dip coating unit. Use a no-clean viscous flip chip flux suitable for dip coating.
The verification test was completed by adding offsets on three different copper pad sizes and three different solder mask openings. Component placement increases the positive and negative offsets in the X direction. All given offsets are measured from nominal positions. Two different test plans for CSP and flip chip were implemented. A total of 450 CSPs and 450 flip chips were placed.
The thickness of the flux layer in the flux unit is 95 microns, which is equal to 50% of the CSP solder ball height. The CSP was first dip coated in the flux unit and then mounted on the test panel with five negative and five positive offset steps. For each offset step, 15 components are placed on the pad layout defined by three different copper foils:
Copper pad = solder ball diameter
Copper pad = 85% solder ball diameter
Copper pad = 70% solder ball diameter
CSP and flip chip assembly using viscous flux (2)