Portions to be soldered of electronic equipment are usually made of Cu. Cu is easily wet by molten solder, and it causes little occurrence of soldering defects such as unsoldered portions and voids when soldering is carried out. However, when portions being soldered made of Cu, such as electrodes of electronic parts such as BGA substrates, are left in the ambient air for a long period after manufacture of the electronic parts, the surface of the Cu is oxidized by the oxygen in the air or sulfurized by exhaust gas of automobiles using fossil fuels or by combustion gas resulting from heating by combustion of fossil fuels. If the Cu surface becomes coated with an oxide or sulfide film as a result of such oxidation or sulfurization, the surface becomes difficult to wet by molten solder at the time of soldering, resulting in the occurrence of soldering defects like those described above.
Therefore, it has been proposed from in the past to use Ni, which has inferior wettability by molten solder compared to Cu but which does not readily form oxides or sulfides when left in ambient air for a long period, for electrodes of electronic equipment. However, since Ni is more expensive than Cu, Ni is not used for entire electrodes and instead Ni is used to apply Ni plating to the surface of less expensive Cu electrodes.
Methods of applying Ni plating to a Cu surface include the electroplating method and the electroless plating method. In the electroplating method, an anode and a cathode are disposed in an electrolytic solution, and a metal plating layer is formed on the surface of the cathode by an electrolytic reaction. However, the electroplating method not only has problems such as the need for special wiring for the purpose of conducting electricity to portions being plated, restrictions on the current density for electrolysis, corrosion of metal portions in the plating apparatus, and the need to use a soluble anode, but it also has problems such as a slow deposition speed of Ni, decreased deposition in locations far from an anode or in recesses, and substantially no deposition of metal on the rear side of a cathode.
In contrast, the electroless plating method has the advantages that Ni plating of uniform thickness is formed over the entire surface of the material regardless of the type of material or its shape by simply immersing a material to be soldered in a plating solution without conduction of electricity. Accordingly, electroless plating is more frequently employed for Ni plating of electrodes of electronic equipment.
A plating solution used for electroless Ni plating is a Ni—P plating solution containing a combination of nickel sulfate as a source of Ni and sodium hypophosphite as a reducing agent. It also contains sodium hydroxide which is used is to maintain the pH of the plating solution constant. An electroless Ni plating layer with a thickness of approximately 0.5-10 μm which is formed with this Ni—P plating solution normally contains approximately 2-15 mass percent of P.
After a BGA substrate has been subjected to electroless Ni plating, solder bumps are formed on the electrodes of the substrate. The BGA substrate is then placed on a printed circuit board, and the bumps are melted to solder the BGA substrate to the printed circuit board. In order to form solder bumps on the BGA substrate, an adherent flux is first applied to the electrodes of the BGA substrate, solder balls are then placed atop the electrodes, and the BGA substrate is heated in a reflow furnace to melt the solder balls. The molten solder balls wet the electrodes of the BGA substrate and are soldered thereto to form solder bumps atop the electrodes. The electrodes of the BGA substrate have been treated in the manner described above with electroless Ni plating containing P. In addition, in most cases, Au flash plating (to a thickness of approximately 0.1-0.5 μm) is applied atop the electroless Ni plating in order to increase the affinity of the surface to solder, and thus the molten solder balls wet the electrodes without producing soldering defects.
In the past, solder used for forming solder bumps on BGA substrates was Pb—Sn solder. When Pb—Sn solder has a composition close to the eutectic composition, i.e., when its Sn content is in the vicinity of 63 mass percent, its melting point is a relatively low level of 183° C. Therefore, it produces little thermal effect on BGA substrates and elements inside the BGA substrates during heating at the time of solder bump formation in a reflow furnace or during heating at the time of subsequent soldering of the BGA substrate to a printed circuit board.
However, Pb—Sn solder contains Pb, which is harmful to the environment, so the use of Pb is not desirable.
Therefore, the use of Pb-containing solder has recently come to be regulated, and so-called lead-free solder which does not contain Pb at all is now being used. Lead-free solder has Sn as a main component, and it is used even for soldering of portions plated by electroless Ni plating.
The below-identified Patent Document 1 states that soldering to portions plated by electroless Ni plating with a lead-free solder has the problems that it results is in a decreased adhesive strength, that the Ni plating layer is dissolved and disappears when reflow is repeated, and that conventional Au flash plating has a plating thickness which is large and makes the adhesive strength inadequate. Therefore, that document proposes that electrodes to which electroless Ni plating has been applied be subjected to electroless gold plating to a thickness of 0.005-0.04 μm as a second metal plating.
Patent Document 1: JP H14-327279 A1