Vapor Phase vs. Convection Reflow in Package-on-Package Technology
By Ryan Wooten, EPIC Technologies
The trend toward smaller, more densely populated electronic assemblies is driving increased use of package-on-package (PoP) technology and electronics manufacturing services (EMS) providers are developing viable manufacturing solutions. Here, I review reliability laboratory analysis on the effectiveness of vapor phase (VP) and convection reflow processing of PoP technology.
Increased use of PoP technology is driven by continued product miniaturization. Smaller products require increased printed circuit board assembly (PCBA) density. Stacking memory is one way to achieve both goals for enhanced functionality and greater packaging density. While PoP was originally adopted by consumer electronics and telecommunications designers, even the medical industry is now designing it into new applications.
That said, PoP technology is not without manufacturing challenges. While the ball grid array (BGA) packages used are well understood, a poor quality process carries a far higher cost in terms of rework or scrap, since two or more BGAs are affected.
Placement requirements and process technology are not significantly different from individual BGAs. Following screen printing, the first component is placed. Then, the second component is flux dipped and placed. In some cases, up to two or more BGAs may be stacked. In many EMS facilities, the only significant line modification is purchasing a flux dip module for placement machines. Thermal profiling during reflow is the same as boards of similar thermal mass.
However, quality needs to be carefully monitored. Any distortion in the bottom component will be reflected in the components above it. Another concern can be excessive heat in convection reflow processes when lead-free components are used. Because the top component is flux dipped, there is no additional solder volume.
Advantages of VP Reflow in PoP Technology
VP reflow technology is not a new process to the EMS industry. It is simply an alternative process for SMT reflow that has been in existence since the early 1970s. The VP reflow process makes use of the heat produced by a boiling fluorinated fluid. This vapor blanket is a uniform temperature zone in which the PCBA solder is reflowed. Heat is transferred to the PCBA as it is immersed into the vapor area until the PCBA reaches temperature equilibrium with the boiling point of the fluid. The primary soldering benefits of VP, in comparison to infrared (IR) or convection, include an oxygen-free (inert) environment without the need for nitrogen, fixed highest temperature exposure, and superior heat transfer on thermally challenging PCBAs.
Vapor phase reflow offers two main advantages for PoP assembly. Better thermal transfer reduces the potential of "potato chipping" or cracking of the components that can occur if the proper thermal mass temperature is not achieved. Also, avoiding a nitrogen requirement for reflow reduces cost.
Analysis of VP Reflow in PoP Applications
In 2009, EPIC's Reliability Lab performed analysis on PoP assemblies reflowed using VP technology. The aim was to verify that solder joint formation between PoP BGAs and the PoP assembly and board pads conformed with IPC standards. The test vehicle used a BGA305 and BGA128 assembly on PCB200-12mmcad (TES-8620) Practical Components testing board. Kester Tacky Flux RF743 was used for the flux dip operation and Henkel no-clean lead-free multicore LF318 Dap88.5 for the solder paste. Five PoP BGA sets were placed and reflowed at 240°C. Electrical testing, X-ray, and cross sectioning were used to assess solder joint quality.
In vapor phase reflow, heat is transferred when the hot, saturated vapor condenses on the PCBA surface and gives up its latent heat of vaporization. The fluid boiling point is the governing factor in peak temperature. The vapor encapsulates the entire surface of the board resulting in the smallest ΔT at short dwell times of the board in the condensing vapor. Thermal transfer is independent of form, color, mass, and mass distribution of the PCBA.
Detailed X-ray inspections were performed on each BGA group, looking for evidence of solder bridges, voids, and open connections. Voids were measured using RINCON measuring software and found compliant with IPC-7095 where the accepted voiding area is less than 25% (the percentage of joint cross-sectional area occupied by the void). Voiding was mainly caused by flux out-gassing within molten solder joints. These bubbles form and pop open when they either grow too large or migrate to the edge of a joint. Upon solidification, the bubbles become voids. While cross-sectioning was not perfectly centered on the BGA balls, the solder joints exhibited in the analysis met all requirements of IPC-610 Revision D for component alignment, voiding, solder ball spacing, and connection (Figure 1).
Cross sections of the components that used Multicore LF318M DAP solder paste together with VP reflow indicate adequate solder volume and uniformity (Figures 2 and 3). The calculated solder volume used is within 5% of the solder volume that should normally occur on a production surface mount line.
Figure 2. A micro-section image showed excellent ball collapse on primary and secondary packages after reflow.
As a surface finish, ENIG offers good solderability, increasingly significant strength, and a considerably improved board finish.
Visual Inspection under 40, 150, 600, and 1200× magnifications resulted in 100% first pass yield (FPY) for all BGAs that used Multicore WS300 Type 4 solder paste under VP reflow.
All 5 BGAs (U1, U5, U8, U11, and U15) were successfully tested for continuity.
All criteria for this process were satisfied with excellent test results.
1. Munroe, C., "Beating the RoHS Heat," Circuits Assembly, March 2008.
Ryan Wooten is EPIC Technologies' engineering manager. Contact him at email@example.com.