Enclosed print heads have recently been available to circuit board manufacturers as an alternate technology to conventional
squeegee printing. By design,enclosed print heads offer several advantages one of which is isolating the solder paste from
the ambient environment thereby stabilizing the rheology of the paste for longer periods. In addition,enclosed print heads
decouple print speed from the pressure within the paste allowing for better control during the print operation.
Very little has been done to demonstrate the advantages of enclosed print heads with regards to print process capability. This
will be the topic of the paper presented here. The study compares the performance of two designs of enclosed print head to
that of a squeegee system on the basis of deposit consistency.

The printing process is always highlighted as contributing the most process defects to a surface mount manufacturing
production facility; typical values presented are around the 60% range. Fact or not it does focus the mind on the
component parts of a mass imaging process especially if it is “hidden” – such as Tooling.
The standard tooling solutions on offer are either fabricated tooling plates or manually located magnetic pins. Both of
these solutions have several drawbacks with regards to set-up time,expense and the inability to support on components.
It is the last issue mentioned which is becoming ever challenging,as substrates become smaller the real estate in which
tooling support can be applied reduces,compound today’s thinner substrates and a real yield risk emerges.
This paper is focused on a new breed of tooling which conforms around the substrate and underside components –
Compliant. To carry out this investigation 3 tooling solutions will be tested,this will include a metal plate to form the
benchmark test. The compliant tooling solutions will be based around modules of pins,which form around the substrates
underside. The first system employs hydraulics to provide compliancy throughout the print cycle. The other solution
utilises pneumatics,which locks the pins after each set-up to form a rigid system.
To investigate all tooling solutions fully a panellised test vehicle fabricated in 0.6mm,1mm and 1.6mm FR4 with pre
assembled SMT components and heavy routing will be utilised throughout.
An automatic laser optic system will be used to measure the solder paste volume,heights and area across the entire
substrate. This data will establish any variation of system to system and indicate which systems have the greatest
flexibility

The research discussed in this paper uses an innovative enhancement technique for printing High Density substrates. The
technique takes advantage of high frequency low amplitude vibrations applied to the stencil at the time of the stencil/substrate
separation. Similar work has been performed in a previous study for the bumping of wafers,1 however the range of pitches
and bump heights addressed in the present work is different. The pitches and bump heights of interest are of the order of 200
and 40 microns respectively. The study shows that enhanced printing produces taller bumps and more uniform bump heights
than can be achieved with conventional printing without vibrations. The study concludes that the vibratory enhanced printing
technology represents an economical and attractive alternative for the mass bumping of High Density substrates.

Recently,wafer bumping using solder paste with very fine solder powder has come into focus as more cost effective than
conventional sputtered or plated methods. This additive method revolves around a stencil printing process similar to
conventional SMT with the exception of the extremely small pitch and desired deposit size. The results and findings of a
print process array of experiments are presented that focus on optimal print deposit area consistency. Variables such as
squeegee type (polymer vs. metal),separation speed and snapoff distance are compared and contrasted.

SPC data has shown that the solder paste printing process is the primary source of soldering defects in SMT assembly.
Consequently,verification of the specified printing properties of solder paste is of paramount importance in the pursuit of
higher quality goals,and higher overall yields. Process variations such as temperature fluctuations in the printing area,
changing printing speeds and varying stencil life have been recognized as important parameters in the characterization of
solder paste,but until today,they never actually have been resolved in a reproducible rheometric methodology.
Moreover,printing developments in recent years,such as the introduction of closed print heads,the apparent diversity in flow
behavior of solder paste on electroform versus laser-cut stencils,and specific adhesion to Au-pads,apparently cannot be
characterized by the traditional single point viscosity measurement that is still used in many facilities today.
Discrepancies in the flow dynamics of different types of solder pastes used in closed print heads clearly show that the vertical
pressure imposed by these systems exerts a major impact on material performance. This phenomenon supports the idea of
characterizing the flow properties of solder paste by controlled shear-stress mode methods rather than by traditional
controlled shear-rate methods.
This paper describes a rheometric QC-routine for solder paste that takes approximately 16 minutes,and is based on the
combination and automation of two different methods. The first method is run in oscillation mode and provides a rheometric
characterization of slumping and tackiness. This procedure is automatically followed by a method that is run in rotational
mode. The latter provides an index (iv) of shear-rate at different settings of controlled stress. The second index (it) provides
viscosity versus different temperature settings,also measured in controlled stress mode. The combined index (ivt) provides a
full indication of the printing properties of a solder paste in one single number. In order to ensure compatibility with
traditional methods,this routine also provides a single point viscosity determination.
The new QC-routine described herein is a fairly quick and cost-effective testing method that yields precise and reproducible
results usable in an SPC program. Moreover,it provides a complete overall picture of the printing properties of solder paste,
including its numeric classification regarding slumping,tackiness,its performance at different speeds and its sensitivity to
temperature variation.

As Electrostatic discharge,humidity can have a bad impact on assembly quality. It requires environmental conditions and
process controls but also risks knowledge. To overcome humidity issue,parts (PCB or components) need to be baked to
remove moisture. Baking drawback is the wettability issue especially with thermal sensitive PCB finishes. Can this
wettability issue introduce reliability defects? Solectron has launched on this topic a project called Aquaboard. This project
mainly deals with the PCB materials behaviours towards humidity,and the impact of different parameters such as baking on
the solder joint reliability. The project took place in two steps. The part 1 is about PCB materials behaviours,mainly the
baking and storage efficiency. Several curves have been drawn for most of the PCB materials (paper,E-glass,aramid,phenol,
epoxy,BT,polyimide,hydrocarbon…): baking curves (from 80° to 120°C),absorption curves (ambient atmosphere,<5%
RH,dry pack storage,85°C-85%RH). The part 2 is a test vehicle to see the impact of baking,assembly process,PCB finish,
atmosphere,design (DFR) on wettability,spreading,voiding,reliability,aluminium bonding…

The SMT (surface mount technology) assembly process for 0.4 mm pitch CSP (chip scale package) components was studied
in this work. For the screen printing process,the printing performance of different solder pastes,aperture shapes and sizes
was investigated. Square apertures and a fine particle size in the solder paste provided a better paste release. Besides
optimizing the printing process capability and minimizing the printing defects such as bridging and missing paste,the total
volume of solder consisting of the paste and the solder ball has to be considered in order to maximize the final process yield.
For the pick & place process,the accuracy required for the placement equipment was determined by studying the selfalignment
of the lead-free CSPs (with Sn/4.0Ag/0.5Cu balls) during the reflow process using lead-free Sn/3.9Ag/0.6Cu paste.
The components were intentionally misplaced up to ~50% off-pad. After reflow,the x-ray inspection showed that the
components had aligned to the pad. By considering the stack-up of the PCB (printed circuit board) pad location and size
tolerances,the solder paste printing tolerances and the placement tolerances,the required alignment accuracy for the pick &
place equipment was established to meet the total process capability requirement.

In order to understand the reaction mechanism of an auto-catalytic type gold plating bath,it is necessary to recognize the
following reactions --- oxidation of reducing agent,gold deposition and dissolution of the underlying nickel deposit.
Therefore,we have been studying the rest potential of the reducing agent,the rest potential of gold deposition and the rest
potential of nickel dissolution,regarding two different types of auto-catalytic gold plating solutions. One is a neutral pH type
non-cyanide auto-catalytic gold plating bath that has been newly developed for PCBs,and the other is an alkaline type autocatalytic
gold plating bath that is currently being used in the ceramic PKG industry. From our evaluation results,we
discovered that a neutral type gold plating bath is apt to dissolve the nickel layer compared to an alkaline type. We also
discovered that this dissolving character depends on the reducing agent type,which is contained in the neutral type autocatalytic
gold plating solution. Furthermore,observation results showed that the characteristics of the immersion gold bath,
which is utilized as a strike gold bath for auto-catalytic gold plating,affect the characteristics of the total deposit. In this
study,we were able to confirm that it is possible to obtain an electroless Ni/Au plating film with excellent wire bondability
and solder joint characteristics by utilizing a newly developed neutral pH auto-catalytic gold plating process.

A novel electrochemical plating process for depositing gold and silver surface finishes using environmentally benign,
organic-based solutions as the plating bath is being investigated. The plating bath solution consists of an extractant and a
diluent of the types used in conventional organic solvent extraction. The organics are very poor electrolytic conductors and
can sustain only short range electrochemical reactions. The deposition mechanism involves the dissolution of a less noble
substrate metal with the simultaneous deposition of more noble metal particles on the surface of the substrate,similar to
immersion plating in an aqueous solution. Feasibility of the concept was demonstrated by loading the organic extractants
with gold or silver in the form of complexed ions. The metal bearing organic liquid was then placed in contact with blank or
patterned copper and nickel surfaces commonly used in the printed circuit board industry. Deposition of a continuous,
adherent gold and silver surface finishes from the organic liquid was achieved with the proper processing conditions. Gold
and silver films were deposited only on the exposed metallic surfaces of the substrate,indicative of a selective area
deposition process similar to immersion plating. Scanning electron microscopy (SEM) indicated that the films were
composed of nanometer sized particles.

Process,Process,Process – these words must ring loudly in our ears,and must be at the forefront of lead free
implementation. The smaller process windows dictated by lead free alloys are going to put greater demands not only on
reflow equipment,but also on first article inspection equipment and procedures. This article will highlight the typical process
concerns associated with lead free alloys with respect to proper first article inspection,qualification of a lead free SMT line,
and adherence to ISO and Six Sigma Quality Management Philosophies. An effective quality control program is completely
dependent on the capability of discovering all possible process problems in order to insure prevention. The enormous
financial and reputation costs of warranty failures faced by leading mass production manufacturers can be greatly reduced by
improving the first article inspection process.
Much has been published concerning the smaller process windows associated with lead free alloys,and their influence on
reliability issues. It is suffice to say that the soldering process up until today has been most forgiving to process problems.
Lead containing solder pastes have known extended soak ranges from 60 to 120 seconds,as well as broad acceptable peak
ranges from 205°C to as high as 255°C. Large temperature deltas in the current soldering process were less problematic for
solderability and reliability. Lead free alloys will see much smaller process windows for soak,and a peak range from 230°C
to 250°C. Working outside these specified process windows will have a negative impact on solder joint integrity,component
safety,and ultimately on reliability.