JPH04250691A - Reflowing method for cream solder - Google Patents

Abstract

PURPOSE:To eliminate dripping or lost shape by preliminary heating at the time of reflowing of cream solder applied to a printed board by printing. CONSTITUTION:After a printed circuit is coated with cream solder by printing, it is so heated to about 150 deg.C by preliminarily heating that a temperature rising speed becomes 1-5 deg.C/sec. Then, when it is heated to a soldering temperature in the case of main heating, the solder applied by printing does not drip, and a bridge or a solder ball is not generated.

Description

[Detailed description of the invention]

0001

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a reflow method for soldering printed circuit boards used in electronic equipment with cream solder.

0002

2. Description of the Related Art Generally, there are two methods for soldering printed circuit boards: a dipping method and a reflow method. The dipping method involves applying flux to the entire surface of the printed circuit board where the soldering parts are located, and then preheating it to around 100℃.
Soldering is performed by bringing the soldered portion of a printed circuit board into contact with molten solder. This dipping method is said to be extremely productive because the soldering processes such as flux application, preheating, and immersion in molten solder are easy, and many points can be soldered at once. It has special features. however,
In the dipping method, flux application and molten solder dipping are performed all at once, so flux and solder adhere to areas where they are not desired, resulting in unexpected accidents such as corrosion and functional deterioration of electronic components. was there.

[0003] In the reflow method, cream solder is applied by printing using a screen, so flux and solder do not adhere to areas other than the soldered areas, and there are no inconveniences as in the dip method. Therefore, the reflow method can be said to be significantly superior to the dip method in terms of reliability.

[0004] In the conventional reflow method, the printed circuit board is heated by first heating the printed circuit board coated with cream solder to about 150°C in preheating, and then increasing the soldering temperature to 220 to 230°C in main heating. Soldering was performed by heating the powder to melt the powdered solder in the cream solder. The purpose of preheating in the reflow method is to alleviate the heat shock that printed circuit boards and electronic components receive during main heating, and to activate the flux. In other words, if printed circuit boards and electronic components are rapidly exposed to high temperatures, they will deform or crack, and if preheating is not sufficient, solderability will deteriorate. Previously, preheating was thought to simply be to alleviate heat shock and activate flux, so it was necessary to maintain an appropriate heating rate, that is, to prevent printed circuit boards and electronic components from receiving heat shock, and to activate flux. Moreover, a heating rate as fast as possible was adopted to increase productivity. Generally, printed circuit boards are reflowed at a temperature increase rate of about 6° C. or more.

[0005]

[Problem to be Solved by the Invention] By the way, in conventional cream solder reflow methods, when soldering printed circuit boards with fine and dense soldering parts, many bridges and solder balls were sometimes observed. . These bridges and solder balls (hereinafter referred to as bridges, etc.)
When we observe the situation in which this happens, we find that the shape of the printed cream solder collapses during preheating, so-called ``sagging'', and this sagging cream solder spreads to areas other than the printed and applied soldering areas, and furthermore. It was found that adjacent cream solder comrades merged and formed a bridge. Generally, it was thought that the cause of the sag that forms bridges and the like was the cream solder itself. It is true that sagging is more likely to occur if the flux content in cream solder is high or if the flux is easily softened. For this reason, attempts have been made to reduce the flux content in the cream solder or to use flux that does not easily soften, but if the flux content is low, the viscosity of the cream solder increases, making screen printing impossible. Additionally, fluxes that do not easily soften generally have poor tacking properties, which weakens the adhesive holding power of electronic components, leading to serious accidents such as electronic components falling during reflow.

SUMMARY OF THE INVENTION An object of the present invention is to provide a reflow method that can reduce dripping of cream solder, which causes bridging, without reducing the amount of flux in cream solder or changing the flux solvent.

[0007]

[Means for Solving the Problems] As a result of intensive investigation into causes of sagging that are not caused by the cream solder itself, the present inventor has discovered that the rate of temperature rise during preheating during reflow has a large effect. The present invention has been completed.

[0008] The present invention uses a reflow method in which cream solder is printed and coated on a printed circuit board, and then preheated and main heated in a reflow oven for soldering. This is a cream solder reflow method characterized in that sec.

[0009] Here, the temperature increase rate of preheating is 1°C/sec.
If it is slower than that, not only will productivity be poor, but the cream solder and printed circuit board will be exposed to high temperatures for a long time, which will cause the powdered solder in the cream solder and the soldered parts of the printed circuit board to oxidize, causing minute solder particles to form. This may cause balls to form or result in unsoldered material to which solder does not adhere. On the other hand, if the temperature increase rate during preheating exceeds 5° C./sec, the temperature of the cream solder increases too quickly, causing the cream solder to sag. The cause of cream solder sag due to the rapid temperature increase during preheating is thought to be that the solvent in the flux liquefies before it evaporates. If the temperature increase rate during preheating is 5°C/sec or less, the solvent in the flux will gradually volatilize without liquefying.
You can suppress sagging.

[0010] Figure 1 shows the experimental results showing the changes in the sagging rate due to differences in the heating rate during preheating, and the experimental results show that the sagging rate increases rapidly when the heating rate exceeds 5°C/sec. I understand that. In this experiment, cream solder was placed on a printed circuit board with a width of 2 mm, a length of 0.2 mm, and a height of 0.
．． The sample was printed to a thickness of 25 mm, and the percentage of the length sagging in the vertical and width directions during preheating was defined as the sag rate.

[0011]

[Example] Spherical powder solder with an average particle size of 50 μm (63
Cream solder consisting of 90% by weight of liquid flux containing 90% by weight of Sn-Pb) and 10% by weight of a solvent with a boiling point of 215°C was applied to the soldered part of a QFP mounting printed circuit board with a lead pitch of 0.5 mm to a thickness of 0.25 mm. Print coating is applied, and the temperature increase rate during preheating of this printed circuit board is 3℃/sec.
Preheating was carried out to 150°C in a reflow oven whose temperature was controlled so that the solder powder was melted and solidified in the cream solder by rapid heating to 230°C, and the reflow was completed. As a result, the cream solder printed and coated on the printed circuit board did not sag during preheating, and no bridging occurred even after reflow.

0012

[Comparative example] Using the same cream solder and printed circuit board as in the example, preheating was performed in a reflow oven at a heating rate of 7°C/sec. As a result, many bridges were generated after soldering.

[0011]

[Effects of the Invention] According to the present invention, since the cream solder printed and applied to the printed circuit board does not sag during the preheating stage, no bridges or the like occur between the soldered parts after reflow, making it a reliable solder. This has an excellent effect that is unprecedented in that a bonded portion can be obtained.

[Brief explanation of the drawing]

FIG. 1 is a graph showing changes in sagging rate depending on temperature increase rate during preheating during cream solder reflow.

Claims (1)

[Claims] Claim 1: In a reflow method in which cream solder is printed and coated on a printed circuit board and then soldered by preheating and main heating in a reflow oven, the temperature increase rate in the preheating is set at 1 to 5°C/sec. A cream solder reflow method characterized by the following.