Bridge Free Wave Soldering of 0.5mm Pitch 100 Pins QFP with SN100C
Bridge-free wave soldering of 0.5mm pitch 100 pins QFP is being consistently achieved with
Nihon Superior’s flagship lead-free solder, SN100C in mass production by a major
manufacturer of hi-fi audio equipment. Bridge-free wave soldering of 0.65mm pitch QFP has
been routinely achieved with SN100C for several years but this breakthrough to the next level
is testimony to the capability of this unique alloy.
SN100C is the lead-free solder invented by Nihon Superior and, as of November 2005,
patented in 24 countries and regions with further applications still being processed. SN100C
is based on the economical and environmentally friendly silver-free Sn-0.7Cu eutectic with
trace additions of Ni and Ge that result in higher fluidity and less oxidation than widely
promoted alternative lead-free solders. It is these properties that have made it possible to
solder successfully fine pitch components.
Since SN100C, like the “63/37” Sn-Pb alloy it replaces, freezes as a true eutectic the fillets
are bright and smooth and free of the deep shrinkage cavities that are characteristic of some
alternative lead-free solders.
Because of its high performance as a solder and the reliability of the joint made with it
SN100C has found wide application in the manufacture of products ranging from simple cell
phone battery chargers through consumer electronics such plasma display panels, DVD
players and domestic appliances and air conditioners through to advanced industrial control
equipment. An increasing number of manufacturers of high-end audio equipment are using
SN100C because of the superior quality of sound produced by systems assembled with this
alloy.
The global electronics market is currently tackling
lead-free issues. Various institutions and research de-
partments continue to evaluate the options, mean-
while many manufacturers of electronic devices and
components have already embraced lead free solde-
ring. At the present time the stabilised tin/copper and
tin/copper/silver (S.A.C.) systems are emerging as
the most popular.
The drawback of a basic SnCu-eutectic alloy is that
an unacceptable number of bridges and short-circuits
result under normal soldering conditions. Unfortun-
ately, this bridge formation cannot be reduced by ad-
justing the process parameters, e.g. by using a more
active flux, a slower processing speed or appropriate-
ly-sized solder pads.
In order to ensure that the electronics industry bene-
fits from the best possible production conditions de-
spite the elimination of lead, Balver Zinn, DKL and
FCT Assembly have joined forces with Nihon Superior
to launch the SN100C range of Lead-Free solders.
Why Clean ‘No Clean’ Flux Residues
For best in class reliability, particularly if the reliability of an assembly is critical, the PCBA
should be be cleaned. Not only to remove flux residues from the PCB surface and solder joints, but also from underneath all the components. Every Manufacturer of PCBA’s has
their own commercial parameters for a specific job, often specified by the end customer.
The assembler should decide early whether to clean, and if so adapt the various processes
available to achieve an assembly that is clean enough to indicate very low levels of ionic
contamination when tested in an Ionograph system. This is the key quality control par-ameter. The level of ionic contamination remains the best indicator of likely long term
reliability of an assembly. One good starting point for making the decision of whether to
clean or not, is to Ionograph test an uncleaned assembly. If significant ionic contamination is
indicated by the test, then cleaning should be considered.
What Contaminants are being removed by cleaning
The contaminants on a PCBA are principally organic in nature, ie natural and /or synthetic
resins, ions, Organic and in some cases small amounts of Halidic acids, solder balls, finger
prints and substrate particulates. The move to lead free alloys has required significant
changes in fluxes and solder pastes to support the necessary higher process temperatures.
The fluxes are usually more active to respond to higher reflow temperatures, which also
presents increased risks. Residues may remain after soldering which have not correctly
decomposed , and can therefore initiate corrosion. If the assembly is to be considered
correctly cleaned, no flux residues or acids, no finger prints, particulates, dust, or cleaning
chemistry residues must remain, including under all components. At the same time, the
whole assembly, including, components, any codings, underfills, adhesive, and the substrate
must not be damaged by the cleaning process or any cleaning chemistry used. The assembly
should be dried and ready for immediate use or test at the end of the cleaning cycle, and
any ink legends must remain untouched and legible.
Why clean ‘No Clean’ Flux residues from PCBA’s
current market statistics indicate that upwards of 70% of all PCBA’s are soldered using products specified as no clean types. This class of flux was developed of course to eliminate
the requirement for further cleaning after soldering. However many assemblers still require
the removal of no clean residues. This can be for a number of reasons. The end use appli-
cation of the PCBA can be the decisive factor. Very high impedence or precision analogue
circuits can have signals affected by the presence of flux residues, even low solids no clean
types. If correctly and consistently heated, the organic acids used in modern no clean fluxes
will completely decompose into harmless components. However any residues may still be
hygroscopic. If these residues absorb moisture from the ambient environment, leakages can
occur between signal pathways. Any flux residues which have been trapped or shielded in
such a way as to cause them not to reach the required temperature for decomposition will
remain a prime source of signal leakage and ultimately corrosion. The risk increases as the
assembly is put under power and over time. One particular source of this problem is flux
residue entrapment between a PCBA and a conveyer transport pallet during wave soldering
operations. Another reason for cleaning can be the requirement for ATE testing of the
finished assembly. No clean fluxes specified as ‘ATE Testable’ tend to leave softer residues to allow the test pins to penetrate and make electrical contact. These residues can accumulate on the test pins which can then in turn require cleaning between test cycles.
The harder residues of newer low solids no clean fluxes are an improvement, but can still
interfere with ATE signal transmission in many instances. It was the problems with ATE
testing which provided the first major requirement for cleaning of no clean fluxes. A further
requirement arises from the requirement to conformal coat PCBA’s. The residues can inhibit
correct adhesion of conformal coatings and may absorb ambient moisture during the manu-
facturing cycle. If the conformal coating is subject to a subsequent curing process, the absorbed moisture can be released causing localized areas of unsatisfactory adhesion and
even separation of the coating from the board. This can allow corrosive materials, conductive particulate or additional moisture to gain penetration into the assembly causing
failures in service,signal transmission problems or component failure. Therefore it is import-
ant to resolve issues concerned with the choice of no clean solder materials and subsequent specification of the cleaning process early in the product definition, particularly with high
reliability assemblies. The correct response can prevent premature PCBA failures in critical
service environments such as defence, aerospace and medical applications.
