REVISTA ROMÂNĂ DE AUTOMATICĂ - IPA SA

REVISTA ROMÂNĂ DE AUTOMATICĂ
35
Because the tombstoning affects especially
the small chip components, the package types
of lead-free chip components used was 0402,
0603, 0805, 1206.
For the experiment lead-free was used , noclean
solder paste OM338T, type 3, (25-
45µm per IPC J-STD-005) from Coockson
Electronics, selected before after special
experiments. The references for the dates of
case dimensions (table 1 – IPC 782) and pads
geometry (table 2) are given in fig. 5a, b.
Tab. 1. Case dimensions
According to geometrical references for cases
and pads, the authors take into consideration
the following hypothesis for stencils
apertures design (table 2): - apertures
dimensions identical with corresponding pads
dimensions; with axial replacement to assure
Smin (Ya4, Xa4); increasing 20% axial and
10% radial (Ya1, Xa1); increasing 10% axial
and 20% radial (Ya3, Xa3,). axial reducing
(Ya1, Xa1), according with:
Xa1 = Wmax, Ya1 = Tmax+2(Cp-Lmin)
(3)
Fig. 5. a. Case dimensions,
b. Pads geometry
In order to assure different volume of solder
paste two stencils were made with different
thickness 0.125mm and 0.150mm according
with apertures from table 4. The authors also
take into consideration one classical solution
for stencils, 0.150mm thickness with 10%
reducing apertures dimensions compared to
pad dimensions and one stencil 0.125mm
without reduction.
Tab. 2. Stencil apertures dimensions
For reflow soldering, the VPS SLC504 equipment from IBL company was used.
4. Results
The solder joints were analyzed according to
the IPC-A-610 D. The solder joints grown
from solder paste deposits follow out stencils
apertures type 1 - 4 shown in figure 6c, d.
The results regarding tombstoning after
thermal profile optimization (figure 7) are
presented in tables 3 and 4.