Traditional SMT assembly is a two dimensional process. Each component is placed on the same horizontal plane in different
X and Y locations. In Package on Package (PoP) assembly,components are placed on successively higher layers. Since
components are stacked on top of each other,traditional solder paste printing cannot be used. The typical SMT method to
print solder paste can only be used to print paste on a single horizontal plane. For PoP,the components that are placed on top
of existing components need to have flux or special dipping solder paste applied at the time of assembly. This paper will explore the challenges and solutions of PoP assembly for the SMT assembly system.

Consideration and selection of dip transfer flux and solder paste for PoP assembly are described,based on process consideration. The crucial properties vital for successful dip transfer include homogeneity,open time on flux/paste bed,volume and consistency of dip transferred material,open time after dip transfer before reflow,and solder joint formation. For each property,one or more practical test methods recommended are described. Overall,this work should provide the assembly house an easy way to select a flux or solder paste adequate for dip transfer PoP assembly applications.

- 3D Manufacturing solutions: Flexible integration
- 3D/SiP Cost of Ownership
- Die vs. Package Stack Analysis
- Smartphone
- Package evolution
- Handheld Processor Roadmap
- PoP
- TMV
- Future Directions for 3D packaging

Examples of various types of parts machined through PCM processes are described.

Examples of various parts machined by PCM processes are featured.

Photochemical machining (PCM) is an excellent method for manufacturing both simple and complex parts in a wide range of
engineering materials. In the harsh economic climate facing manufacturing industry today,it is important to utilise costeffective
processes to produce high-quality parts rapidly for just-in-time delivery. This paper discusses the best methodologies to utilise PCM to its full extent as an extremely versatile process within the electronics,electrical and mechanical engineering disciplines.

Lead free soldering with no clean solder pastes represent nowadays the most common process in electronic assembly. A solder paste is usually considered as no-clean if it passes all IPC J-STD-004 corrosion tests: copper mirror,copper panel corrosion test,Surface Insulation Resistance (SIR) and Elecrochemical Migration (ECM). Other SIR and ECM tests are described in Bellcore GR-78-CORE and JIS Z3197 standards.
Although SIR and ECM tests are recognized by all standards authorities to evaluate the solder paste residue corrosivity after reflow,a more selective method,the Bono test,has been developed and implemented in some French companies as a qualification criterion. It has been proven that compared to common corrosion tests,the Bono test better differentiates the nature of solder paste residues.

Electrochemical migration (ECM) is the growth of conductive metal filaments on a printed circuit board (PCB) through an electrolyte solution under a DC voltage bias. ECM can cause a reduction in surface insulation resistance (SIR) between adjacent conductors,generate a path of leakage current,and lead to intermittent or catastrophic circuit failures.
Solder mask has been widely used on printed circuit boards to define wettable surfaces,reduce moisture access,control outer layer impedance,and improve corrosion resistance. The mechanical and thermal properties of solder mask have been widely reported,but systematic studies of their influence on ECM have been few. This paper presents the results of temperature-humidity-bias (THB) testing of more than 1000 hours duration at 40V,65°C,and 88% relative humidity for comparative evaluation of ECM on circuit boards with and without a solder mask. The boards were HASL finished and wave soldered using a no-clean,low solids flux. Besides primarily assessing the effects of using a solder mask on ECM,the effects of solder alloy composition (eutectic SnPb versus Sn-3.0Ag-0.5Cu) were also investigated. In situ monitoring of SIR was performed throughout these tests. Optical microscopy and scanning electron microscopy were employed to examine the correlation between the physical attributes of dendrites and the measured SIR,as well as to evaluate the effects of solder mask and solder alloy on ECM. Ion chromatography (IC) was conducted to measure contaminant levels on the surface of the PCBs. Elemental mapping by energy dispersive X-ray spectroscopy was employed to identify the migrating species and their distributions and morphologies within the dendrites. As expected,the use of a solder mask resulted in higher SIR,but a dramatic difference was observed in its effect on dendritic growth and characteristic life.

Many electronics manufacturers perform SIR testing to evaluate solder materials and sometimes the results they obtain differ
significantly from those stated by the solder material provider. The difference in the results is typically the result of SIR coupon preparation. This paper will discuss the issue of SIR coupon preparation,board cleaning techniques,and how board cleanliness directly affects SIR results.
We will also discuss the differences between two of the industry’s most common SIR specifications J-STD-004A and -004 B. The subjective nature of SIR testing on water washable/soluble materials will also be reviewed. In addition,testing for SIR
when the assembly process has multiple residue sources,such as paste,wave solder flux,rework flux,etc.,will be considered.

Using embedded structures in printed circuit boards,changes in moisture content have been monitored using route-impulse-energy measurement,capacitance measurement and weight gain. All three methods showed good correlation for different prepregs with similar structures.
Two designs of capacitor plate,a solid plate and a multi slotted plate,showed significantly larger percentage increases in capacitance values (10%) than the corresponding mass increases (0.35%) from moisture ingression effects. A good correlation was also obtained between the RIE measurement results and the board mass changes The RIE values showed significantly larger percentage increases (20%) than the corresponding mass increases (0.35%) over a similar timeframe.
Although some results indicated a slightly greater moisture take-up for the 2116 prepreg samples compared to the 7628 prepreg samples,the majority of measurements did not show significant performance differences between the two prepreg types.
Both the RIE measurement and capacitance methods show promise in evaluating the moisture content of printed circuit boards. Capacitance measurements using capacitors of a solid plate design,have the disadvantage of inhibiting the take up of moisture if the plates are near the surface of the PCB. RIE coupons have an aspect ratio which enables them to be relatively easily incorporated into the break-off panel that commonly occurs around the outside of many PCB designs during manufacture. Provided that similar coupons had been characterised in the dry condition previously,and that the PCB has the same build characteristics,both types of coupon could be interrogated immediately prior to assembly,to determine the current level of moisture content.