A. Specification Summary
The specification of the press-fit connector we developed is
summarized in Table II.
In Table II, "Size" means the male contact width (the so-called "Tab Size") in mm.
B. Appropriate Contact Force Range Determination
As the first step of press-fit terminal design, we must
determine the appropriate range of contact force.
For this purpose, the deformation characteristic diagrams of
terminals and through-holes are drawn schematically, as shown
in Fig. 2. It is indicated that contact forces are in a vertical axis,
while terminal sizes and through-hole diameters are in the
horizontal axis respectively.
C. Minimum Contact Force Determination
The minimum contact force has been determined by (1)
plotting the contact resistance obtained after the endurance
tests in vertical axis and the initial contact force in horizontal
axis, as shown in Fig. 3 schematically, and (2) finding the
minimum contact force as ensuring the contact resistance being
lower and more stable.
It is hard to measure the contact force directly for the pressfit connection in practice, so we obtained it as follows:
(1) Inserting terminals into through-holes, which have
various diameters beyond the prescribed range.
(2) Measuring the terminal width after insertion from the
cross section cut sample (for example, see Fig. 10).
(3) Converting the terminal width measured in (2) into the
contact force using the deformation characteristic
diagram of the terminal obtained actually as shown in
Fig. 2.
Two lines for the terminal deformation mean ones for
maximum and minimum terminal sizes due to dispersion in
manufacturing process respectively.
Table II Scecification of the Connector we develored
It is clear that the contact force generated between
terminals and though-holes is given by the intersection of two
diagrams for terminals and through-holes in Fig. 2, which
means the balanced state of terminal compression and throughhole expansion.
We have determined (1) the minimum contact force
required to make the contact resistance between terminals and
though-holes lower and more stable before/after the endurance
tests for the combination of minimum terminal sizes and
maximum through-hole diameter, and (2) the maximum force
sufficient to ensure the insulation resistance between adjacent
through-holes exceeds the specified value (109Q for this
development) following the endurance tests for the
combination of maximum terminal sizes and minimum
through-hole diameter, where the deterioration in insulation
resistance is caused by the moisture absorption into the
damaged (delaminated) area in PCB.
In the following sections, the methods used to determine
the minimum and maximum contact forces respectively.
D. Maximum Contact Force Determination
It is possible that interlaminar delaminations in PCB induce
the lowering of insulation resistance at high temperature and in
a humid atmosphere when subject to excessive contact force,
which is generated by the combination of the maximum
terminal size and the minimum through-hole diameter.
In this development, the maximum allowable contact force
was obtained as follows; (1) the experimental value of the
minimum allowable insulation distance "A" in PCB was
obtained experimentally in advance, (2) the permissible
delamination length was calculated geometrically as (B-CA)/2, where "B" and "C" are the terminal pitch and the
through-hole diameter respectively, (3) the actual delamination
length in PCB for various through-hole diameters has been
obtained experimentally and plotted on the delaminated length
vs. initial contact force diagram, as shown in Fig. 4
schematically.
Finally, the maximum contact force has been determined so
as not to exceed the allowable length of delamination.
The estimation method of contact forces is the same as
stated in the previous section.
E. Terminal Shape Design
The terminal shape has been designed so as to generate
suitable contact force (N1 to N2) in the prescribed through-hole
diameter range by using three dimensional finite element
methods (FEM), including the effect of pre-plastic deformation
inducing in manufacturing.
Consequently, we have adopted a terminal, shaped like an
"N-shape cross section" between the contact points near the
bottom, which has generated an almost uniform contact force
within the prescribed through-hole diameter range, with a
pierced-hole near the tip allowing the damage of PCB to be
reduced (Fig. 5).
Shown in Fig. 6 is an example of the three-dimensional
FEM model and the reaction force (i.e., contact force) vs. the
displacement diagram obtained analytically.
F. Development ofthe Hard Tin Plating
There are various surface treatments for preventing the
oxidization of Cu on PCB, as described in II - B.
In the case of metallic plating surface treatments, such as
tin or silver, the electrical connection reliability of press-fit
technology can be ensured by the combination with
conventional Ni plating terminals. However in the case of OSP,tin plating on the terminals must be used for to ensure long term electrical connection reliability.
However, conventional tin plating on terminals (for
example, of 1ltm thickness) generates the scraping-off of tin during the terminal insertion process. (Photo. "a" in Fig. 7)
and this scraping-off probably induces short-circuits with adjacent terminals.
Therefore we have developed a new type of hard tin
plating, which does not lead to any tin being scraped-off and which ensures long term electrical connection reliability simultaneously.
This new plating process consists of (1) extra thin tin
plating on underplating, (2) a heating (tin-reflow) process,
which forms the hard metallic alloy layer between the
underplating and the tin plating.
Because the final residue of tin plating, which is the cause
of scraping-off, on terminals becomes extremely thin and
distributes non-uniformly on the alloy layer, no scraping-offof tin was verified during the insertion process (Photo "b" in Fig. 7).
Post time: Dec-08-2022