Metal treatment – Compositions – Fluxing
Reexamination Certificate
2001-10-04
2002-05-21
Jenkins, Daniel J. (Department: 1742)
Metal treatment
Compositions
Fluxing
Reexamination Certificate
active
06391123
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to a solder paste suitable for use in reflow soldering of electronic parts mounted on printed circuit boards. More particularly, it is concerned with a solder paste made of a lead-free solder alloy of an Sn—Zn—Bi ternary system.
2. Description of the Related Art
Electronic parts for use in electronic appliances can be mounted on printed circuit boards either by insertion mounting technology (IMT) (also called through hole mounting technology or TMT) or surface mounting technology (SMT).
Electronic parts to be mounted by IMT comprise electronic elements having elongated rod-like or pin-like leads projecting therefrom. They are normally soldered by the flow soldering technique. In a typical process of IMT, electronic parts are mounted on one surface of a printed circuit board by inserting the leads of each part into through holes formed in the board until the leads pass through the holes, and the leads are secured to lands formed on the opposite surface of the board around the through holes by flow soldering, which includes the steps of application of a flux, preheating, contact with a molten solder, and cooling.
IMT is disadvantageous in that there is a limit to the extent to which the overall size of the electronic parts, including leads, can be reduced, particularly in the case of discrete single function parts such as resistors and capacitors. Another problem of IMT particularly encountered with transistors and pin grid arrays (PGA's) having a great number of leads is that it is difficult to insert all the leads completely into the through holes of the board, and some leads may be bent by failure to be in alignment with the through holes.
In view of these problems of IMT, SMT has recently been employed widely. Electronic parts to be mounted by SMT are called surface mount devices (SMD's) and they include chip-type, discrete, single-function parts such as resistors and capacitors as well as IC packages such as quad flat packages (QFP's) and small outline packages (SOP's).
SMD's are soldered on the same surface of a printed circuit board on which they are mounted without their leads being inserted through holes. Therefore, SMD's are not suitable for soldering by flow soldering, in which the areas to be soldered must be contacted with molten solder. If SMD's are soldered by flow soldering, they may come into direct contact with the molten solder, which may result in thermal damage thereto, or the molten solder may be prevented from attaching to all the areas to be soldered due to the uneven surface of the printed circuit board.
For this reason, most SMD's are soldered by reflow soldering. In a typical process of SMT employing reflow soldering, a solder paste which is comprised of a solder alloy powder uniformly mixed with a flux is applied to the surface of a printed circuit board on which SMD's are to be mounted by printing or dispensing. Thereafter, SMD's are disposed on that surface of the printed circuit board, and the board with the SMD's disposed thereon is heated in a reflow furnace to a soldering temperature sufficient to allow the solder alloy powder to melt in order to perform soldering. The above-described problems of flow soldering when applied to soldering of SMD's can be eliminated by reflow soldering.
Reflow soldering causes the entire printed circuit board to be heated along with the electronic parts (SMD's) disposed thereon. In order to minimize adverse thermal effects of this heating on the printed circuit board and SMD's and prevent bumping (sudden splashing) of the solder paste during heating, the reflow furnace may be designed such that preheating is performed at a temperature in the range from 100 to 170° C. before main heating to the soldering temperature, thereby minimizing the duration of main heating.
The soldering temperature, which is the peak temperature for main heating, is normally from 20 to 40° C. above the liquidus temperature of the solder alloy powder used in the solder paste in order to completely melt the powder, and it varies to a certain extent depending on the size and thickness of the printed circuit board and the packing density of the electronic parts mounted thereon. The soldering temperature is recommended to be as low as possible in order to minimize adverse thermal effects particularly on the electronic parts disposed on the board. Therefore, it is preferable to use a soldering alloy powder having a low liquidus temperature in order to lower the soldering temperature.
The solder alloy powder used in a conventional solder paste is a powder of an Sn—Pb solder alloy, particularly an Sn—Pb eutectic solder alloy, in view of its low melting temperature of 183° C. and good solderability. Soldering with a conventional solder paste prepared from an Sn—Pb eutectic solder alloy powder is generally performed at a temperature of 230° C. or below after preheating to minimize the duration of main heating. Such soldering conditions are generally sufficient to avoid thermal damage to SMD's and printed circuit boards during reflow soldering.
When electronic appliances are to be discarded, they are typically disposed of in landfills, where they may be brought into contact with rain, which has recently become acidic. Such acid rain causes the Sn—Pb alloy solders used in discarded electronic appliances present in landfills to dissolve and contaminate groundwater. If groundwater contaminated with lead is ingested by a person for many years, the accumulation of lead in the person's body may result in lead poisoning. For this reason, it has recently been recommended to use a lead-free solder alloy in the electronics industry. This recommendation also applies for a solder paste for use in reflow soldering.
Typical lead-free solder alloys are Sn-based alloys comprising a major proportion of Sn and a small amount of at least one additional element such as Ag, Cu, Bi, Sb, and Zn.
Sn—Ag alloys have a eutectic composition of Sn-3.5 Ag with a melting temperature of about 220° C. Even if this composition, which has the lowest melting temperature among Sn—Ag alloys, is used as a solder alloy, the soldering temperature will be as high as 250° C. or above, which may cause thermal damage to electronic parts during reflow soldering. Addition of a small amount of Bi and/or In to an Sn—Ag alloys can decrease the solidus temperature of the alloy, but its liquidus temperature is not decreased significantly, so the soldering temperature still remains high. Sn—Ag alloys have another problem of poor surface gloss of soldered joints formed therefrom, leading to a decrease in attractiveness of the soldered products.
Sn—Cu alloys have a melting temperature of 227° C. for a eutectic composition of Sn-0.7 Cu, which makes the soldering temperature too high to avoid thermal damage to electronic parts during reflow soldering. These alloys have another problem of poor solderability. The addition of Bi and/or In is not effective to significantly decrease the liquidus temperature of these alloys, as is the case with Sn—Ag alloys.
Sn—Bi alloys have a very low melting temperature of 139° C. for a eutectic composition of Sn-57 Bi. Therefore, they make it possible to perform reflow soldering even at a lower temperature than the conventional Sn—Pb eutectic solder alloy with no concern about thermal damage to electronic parts. However, such alloys are very brittle due to the presence of a large proportion of Bi, and the resulting soldered joints are readily detached when subjected to even a mild mechanical impact.
Sn—Zn alloys have a eutectic composition of Sn-9 Zn with a melting temperature of 199° C., which is low enough to make it possible to perform reflow soldering at a soldering temperature of 230° C. or below to minimize thermal damage. The alloying element Zn or zinc is harmless to human bodies, and it is an abundant and inexpensive element. Therefore, Sn—Zn lead-free solder alloys are advantageous from the viewpoints of safety
Kurata Ryoichi
Nakamura Sinzo
Takahashi Hiroshi
Jenkins Daniel J.
Senju Metal Industry Co. Ltd.
Tobias Michael
LandOfFree
Solder paste does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Solder paste, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Solder paste will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-2880623