PoP Assembly

PoP Assembly Challenges

• The widely adopted process for PoP assembly is single pass reflow of the top and bottom packages after being assembled on the PCB.

• The solder balls of the top package are either dipped in flux or solder paste, before being placed on the top of the bottom BGA package during assembly

• The flux or paste dip will be unable to accommodate for the solder ball co-planarity issues and package warpage between the top and bottom packages.

PoP Reflow Challenges

• The solder balls dislodging or migrating off the pad with improper reflow profiles and excessive package warpage, leading to solder ball bridging, solder slumping, head and pillow defects, or open joints.

• Substrate warpage characterization for the component packages and PCBs are becoming more critical as geometries become smaller and substrates become thinner. Compounding this issue is the use of lead free solder, which melts at higher temperatures, increasing the coefficient of thermal expansion (CTE) mismatch and, hence, the package warpage.

• Experimental studies show that the root cause for more than 90 percent of the PoP assembly defects is package warpage.

• Minimizing package warpage is possible by – Controlling reflow time– Temperature– Packaging materials – Selecting appropriate package design.

• From the PCB assembly perspective, the first two parameters gain importance. Arriving at an acceptable reflow profile that falls within the specifications can control these parameters.

Causes for package warpage• Package warpage is caused due to the

mismatch in CTE between the substrate, mold compound (package overmold) and the die (figure 2a). Typically, higher glass transition temperature (Tg) mold compounds are used to balance the coefficient of thermal expansion (CTE) mismatch between the die and the substrate. The effect of the mold compound becomes negligible at reflow temperatures, as they tend to become soft. The periphery of the bottom package that accommodates the balls of the top package is devoid of any mold compound. Therefore, at reflow temperature, the exposed thin substrate will expand more when compared to the substrate under the mold compound, resulting in a smiling (concave) warpage of the bottom package (figure 2b). On the other hand, the top package that has the mold compound extending to its edges experiences a crying (convex) warpage.

Defects and root causes

• The defects that are caused during the reflow processand the possible root causes are identified in table 1

• Solder on Pad PoP technology Solder on Pad (SOP) PoP assembly technology addresses several issues, like standoff height, solder joint collapse size and the ability to solder fine pitch components. In SOP technology, solder balls are present on the topside of the bottom package. These balls must be aligned with the balls of the top package during assembly. The ball-to-ball stacking (figure 3) provides a larger solder joint collapse compared to non-SOP based PoP. This large collapse helps to absorb package warpage. On the other hand, the ball-toball stacking presents new challenges, particularly in the reflow process. The most common challenge is the balls of the top package sliding off from the balls of bottom package and the inability to create a good solder joint from the twocontacting solder spheres.

Future challenges

• The technology trend towards package miniaturization necessitates – thinner substrates, – reduced standoff height, – smaller die, – thinner encapsulant materials etc.

• While all these design parameters help achieve better assembly yield, they present additional challenges in the assembly, especially during the reflow process. Reworking PoP assemblies after the reflow process also requires a greater level of operator skill, therefore requiring the first pass assembly yield to be high.

Conclusion

• The extent of PoP warpage during reflow dictates the final package stacking yield on the PCB.

• The main objective of a good reflow profile must be to control the package warpage throughout the process.

• Since the use of lead free solder demands a narrow process window in the reflow process, more emphasis should be placed on arriving at an acceptable reflow profile.

• For extended reliability, different lead free solder alloy combinations are also being considered for the top and bottom packages, making reflow soldering the most important process for achieving good yields.

Tools

• Solder jet printing technology

PoP Topics• What is Package on Package (PoP)?• Benefits of PoP Stack Packages• Component Standards• Component Types• JEDEC Standards• PCB Design Rules• Pad Layout• Via Hole Connection• Lead-Free Assembly• Engineering Interviews• Stencil Printing• POP Placement• Tack Flux• Dip Solder Paste• Reflow Soldering• Convection• Vapour Phase• Temperature Profiling• Inspection• Optical Inspection• X-ray inspection• Underfill• Rework• Package on Package Defects

• Optical inspection of joints• X-ray inspection• Component alignment• Solder paste dipping• Liquid flux application• Paste & flux depth monitoring• Open solder connections• Solder balling• Package warpage• Excess paste• Excess flux• BGA voiding• Package cracking• Mask damage

Package-on-Package (PoP) Assembly Materials

• PoP Fluxes are suitable for stacking CBGAs and small PBGAs, which are not subject to flexing.

• PoP Pastes are useful when board flexing may occur during reflow, such as larger PBGAs. The presence of a small amount of metal powder allows complete reflowed joints to form, even when the substrate twists and gaps form between the solder sphere and the substrate

Key Performance Needs of PoP Assembly Materials

• Particle-, Crystal-, Bubble-Free • Homogeneous • Viscosity stable with time • Pb-Free alloy compatible • Applied by dipping • Air reflow • Wetting onto a variety of metals • No-clean materials

PoP Assembly

Terminology

• PSvfBGA – Package Stackable Very Thin Fine Pitch

BGA

• PSfcCSP– Package Stackable Flip Chip CSP

• TMV® PoP– Through Mold Via Package on Package

Dip pastes may elevate solder ball levels

• Dip paste has a lower metal content, often designed for nitrogen reflow, and has a smaller solder particle size than stencil printing grades.

• Hence, it is more prone to solder balling and slump during reflow. Based on this, the solder balling would be more likely on the surface of the PoP devices than the surface of the board.

Study on PoP Assembly

• The top and bottom devices in the PoP stack are offered with a range of different alloys.

• different ball alloy variations – – a SAC305 stack– a SAC405 stack– a stack with a SAC105 upper package, and a

SAC125 lower package.

• different dippable materials – dip fluxes – dippable solder pastes

• SAC305 – Type V powder and Type VI powder

• PoP (PACKAGE ON PACKAGE): AN EMS PERSPECTIVE ON ASSEMBLY, REWORK AND RELIABILITY  Heather McCormick, et al.Celestica Inc.

• Price: $10

• http://smta.org/knowledge/article_cart.cfm

Reflow Errors

• floating QFNs

• excessive solder

• short and opens

Machine

• MYDATA MY500– Solder Jet Printing– 1.8 million dots per hour

BGA Variants• CBGA and PBGA denote the Ceramic or Plastic substrate material to which the array is

attached. • CABGA: Chip Array Ball Grid Array • CTBGA: Thin Chip Array Ball Grid Array • CVBGA: Very Thin Chip Array Ball Grid Array • DSBGA: Die-Size Ball Grid Array • FBGA or Fine Ball Grid Array based on ball grid array technology. It has thinner contacts

and is mainly used in system-on-a-chip designs. Known as FineLine BGA by Altera • FCmBGA: Flip Chip Molded Ball Grid Array • LFBGA: Low-profile Fine-pitch Ball Grid Array • UFBGA and UBGA and Ultra Fine Ball Grid Array based on pitch ball grid array. • MBGA: Micro Ball Grid Array • PBGA: Plastic Ball Grid Array • MCM-PBGA: Multi-Chip Module Plastic Ball Grid Array • TEPBGA: Thermally Enhanced Plastic Ball Grid Array • SuperBGA (SBGA): Super Ball Grid Array • TABGA / TBGA: Tape Array BGA • TFBGA or Thin and Fine Ball Grid Array

• Solder Balling

My Conclusion

• Dip Paste is better for PoP Assembly

• Lead-Free Electronics: iNEMI Projects Lead to Successful Manufacturing– By Edwin Bradley, Carol A. Handwerker,

Jasbir Bath, Richard D. Parker, Ronald W. Gedney

– John Wiley & Sons