JP2915543B2 - How to set reflow conditions for reflow furnace - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to a method for setting reflow conditions of a reflow furnace.

(Conventional technology and problems to be solved) With the advance of microelectronics, soldering technology as a joining technology has been applied to minute and fine members, so-called micro soldering has become very important. Is coming. Recently, techniques for precision soldering and soldering of minute parts are rapidly developing, and at the same time, high production efficiency is required for automation and labor saving. Under these circumstances, a reflow soldering method has recently been adopted as a new soldering technique in which solder is supplied in advance to the soldering location, and this is melted using a heat source such as hot air, infrared rays, or a laser for soldering. Is coming.

With this reflow soldering method, chip components have been used in place of conventional leaded components with the miniaturization of electronic devices, and surface mounting methods have been introduced to further increase the mounting density. For this reason, it has become widely used because it has become necessary to accurately and efficiently join small soldered portions.

In addition, the reflow soldering method has the following advantages: (I) an appropriate amount of solder having an appropriate composition can be supplied only to a necessary place, (II) no contamination by impurities that cannot be avoided by the immersion soldering method, and (III) surface mounting. No need for through-holes to be possible. (IV) There are many features such as the need for solder resist processing.

A reflow furnace for performing reflow soldering includes a preheating zone for heating a lead portion of a component mounted on a substrate to a predetermined temperature, and a uniform heating for uniformly heating a lead portion of the component heated in the preheating zone. Each zone such as a zone, a main heating zone for rapidly heating a lead portion uniformly heated to a predetermined temperature in the heat equalizing zone to a predetermined temperature at which the solder melts and performing a fusion bonding, and components were mounted. It is constituted by a conveyor or the like which places a substrate and sequentially passes through a preheating zone, a soaking zone, and a main heating zone at a predetermined transport speed.

By the way, conventionally, the setting of the heater temperature of each heating zone of the reflow furnace for a new board is performed by attaching a thermocouple to the soldered circuit board and setting the heater temperature based on the experience and intuition of the worker so that the target temperature history (profile) is obtained. The reflow condition is set by changing the temperature setting several times.

However, in the above-described conventional method for setting the heater temperature, since the reflow condition is set based on the experience and intuition of the operator, it takes a very long time to set the reflow condition, and the workability is extremely poor. This is especially true when an inexperienced operator sets reflow conditions.

SUMMARY OF THE INVENTION The present invention has been made in view of the above points, and has been made in consideration of the above-described circumstances. A reflow furnace capable of setting reflow conditions for a new substrate by changing a conveyor speed several times without relying on the experience and intuition of an operator. An object is to provide a method for setting conditions.

(Means for Solving the Problems) According to the present invention, in order to achieve the above object, when setting reflow conditions for a new substrate in a reflow furnace, the first heater temperature setting of the reflow furnace is performed in advance by setting data. The obtained coefficient, the conveyor speed, the ratio of the heat capacity and the heat transfer area of the substrate and the function of the heater temperature represented by the target temperature of the substrate are used, and the second measurement is performed using the measurement result. Set the reflow conditions for the third time, set the reflow conditions for the third and subsequent times, and set the data of the first conveyor speed and the maximum temperature of the lead of the board and the mounted parts, and the second conveyor speed and the board and the mounted parts. Is obtained by linear approximation from the data of the maximum temperature of the lead, and the conveyor speed is set so that the lead of the mounted component reaches the target maximum temperature.

(Operation) The first setting of the heater temperature of the reflow furnace is represented by a coefficient obtained by taking data in advance, a conveyor speed, a ratio of the heat capacity of the substrate to the heat transfer area, and a target temperature of the substrate. This is performed using a function of the heater temperature. Then, the first measurement of the conveyor speed and the maximum temperature of the lead of the board and the mounted component is performed, and the second reflow condition is set using the measurement result. The reflow conditions after the third time are set by linear approximation from the data of the first time of the conveyor speed and the maximum temperature of the lead of the board and the mounted parts, and the second time of the data of the conveyor speed and the maximum temperature of the board and the lead. Ask. Then, the conveyor speed is set so that the leads of the mounted components reach a target maximum temperature. Set the reflow conditions of this reflow furnace at least 3
It can be performed in a single operation.

Embodiment An embodiment of the present invention will be described below in detail with reference to the accompanying drawings.

In FIG. 1, a reflow furnace 1 includes a preheating zone I, a soaking zone II, a main heating zone III, a belt conveyor 2 for transporting a substrate, and the like, and includes a preheating zone I, a soaking zone II, and a main heating zone. III are arranged horizontally in a straight line, and the belt conveyor 2 is arranged horizontally along each of these zones I to III.

The reflow furnace 1 has an infrared heating method (IR method) in the preheating zone I,
The hot air method (convection method) is adopted for II and the infrared and hot air method is adopted for the main heating zone III.

The preheating zone I is composed of two surface heaters 3 and 4 facing each other at a predetermined interval. Soaking zone II
It is composed of two air heaters 5 and 6 and ducts 7 and 7 arranged side by side along the conveying direction of the belt conveyor 2. The main heating zone III includes two surface heaters 8 and 9 and an air heater 10 which are opposed to each other at a predetermined interval. Hot air discharge port 11a of duct 11 of air heater 10
Is on the loading side of the soaking zone III, and the suction port 11b is the main heating zone
Each of the ducts 11 is provided with a heater core 12 and a blower 13 on the carry-out side of the III so as to open toward the belt conveyor 2.

The belt conveyor 2 is driven by a transfer motor 15 in a direction indicated by an arrow A, and transfers the placed substrate 20 through a preheating zone I, a soaking zone II, and a main heating zone III at a predetermined transfer speed (hereinafter referred to as “the speed”). Conveyor speed). Each of the surface heaters 3, 4, 8, and 9 is a temperature control device (not shown).
And the temperature thereof can be adjusted.
Numerals 6 are respectively connected to a hot air source (not shown), and the temperature of the hot air can be adjusted by the temperature control device. Further, the heater core 12 of the air heater 10 is also connected to the temperature control device. The temperature of the hot air from the air heater 10 is fixed at a predetermined temperature as described later. Then, the hot air from the air heater 10 circulates through the main heating zone III to have a uniform temperature.

Hereinafter, a method of setting the heater temperature of the reflow furnace 1 will be described.

First, the heating of each of the heating zones I to III of the reflow furnace 1 will be described. Preheating zone I as shown in FIG. 2, respectively Th ₁ each temperature of each surface heater 3,4,8,9 of the heating zone _{III, Th 2, Th 3,} Th 4, the air heater of the soaking zone II 5 , 6 as Tair ₁ and Tair ₂ , and the temperature of the air heater 10 in the main heating zone III as Tair ₃ .

The temperature Tair _{3 of the} air heater 10 is used at about 250 ° near the maximum temperature, and Tair ₁ and Tair ₂ are made equal to the target substrate temperature Taf immediately before the main heating. The adoption of the hot air system in the soaking zone II has the advantage that the temperature can be uniformly increased without increasing the temperature above the set temperature.

Therefore, the remaining heater temperatures are set to the temperatures Th ₁ , Th ₂ , Th ₃ , and Th _{4 of the four} surface heaters ₃ , _{4, 8,} and 9. Then, the temperature Th ₁ and Th ₂ surface heaters 3 and 4, is set equal to a temperature Th ₃ and Th ₄ faces the heater 8 and 9.

Further, as shown in FIG. 3, the surface temperatures of three points of the substrate 20, the component 21 mounted on the substrate 20, and the lead 22 of the component 21 are measured by thermocouples 23, 24, and 25. The thermocouple 23 measures Ta at a point a where there is no component or the like in the vicinity and at a point a where the maximum temperature of the substrate 20 is expected. Thermocouple 24
Is the component with the largest heat capacity among the components mounted on the board 20.
The temperature Tb of the point b of the lead 22, ie, the lead whose temperature is least likely to rise, is measured. The thermocouple 25 measures the temperature Tc at the point c of the component 21 having the maximum heat capacity. These temperatures are measured and recorded by a temperature recording device (reflow checker) 26, for example. Then, as described later, the temperature profiles of these three points a to c are measured as shown in FIG.

Next, a method of setting the heater temperature will be described with reference to the flowcharts shown in FIGS. 5 (A) to 5 (D).

First, the target lead temperatures Tbm (for example, 210 ° C.) and Tbf (for example, 160 ° C.) of the point m (the main heating zone III) and the point f (the soaking zone II) of the temperature profile shown in FIG. 4 are input. (Step 1). Other conveyor speed CS (for example, 0.7m / min), heat capacity per unit heat transfer area of substrate 20
Enter C / A. The heat capacity C of the substrate 20 is represented by the product (c × V) of the specific heat c of the substrate 20 and the volume V, and the volume V is the product of the heat transfer area A of the substrate 20 and the plate thickness t (A × t). ). Therefore, the heat capacity per unit area (hereinafter simply referred to as “heat capacity”) C / A is C / A = (cAt) / A = ct,
It is expressed as a function of the thickness t of 0. The specific heat c is the amount of heat (kJ / kg) required to raise the temperature of a unit weight of a homogeneous substance by 1 ° C.
° C). Therefore, the heat capacity C / A of the substrate 20 is
For example, specific heat c of glass epoxy resin (= 0.8kJ / kg ℃)
And its thickness t (for example, 1.6 mm) (C / A = ct = 0.
8 × 1.6).

The initial conveyor speed CS is set in the range of 0.6 to 1.0 m / min.

Next, the temperatures Tak, Taf, and Tam of the preheating zone I, the soaking zone II, and the main heating zone III of the substrate 20 are estimated (step 2). One criterion on the substrate 20 is the temperature of the lead 22 of the maximum heat capacity component 21 (this temperature requires a minimum temperature of the melting point of the solder + α ° C. at the point m.
It is necessary to keep the temperature low in order to prevent oxidation of 20 and not to raise the temperature of other parts too much).

However, the temperature of the lead 22 depends on the component temperature rather than the substrate temperature, and there are many types of components, and the lead portion 22 may float above the melting point of solder, making it unreliable to create a database. It is necessary to set the heater temperature based on the substrate temperature. So, Ta
The temperatures k, Taf, and Tam are set, for example, as follows. Tak = Tbf + 5 ° C = 160 + 5 = 165 ° C, Taf = Tbf + 5 ° C
= 165 ° C, Tam = Tbm + 15 ° C = 210 + 15 = 225 ° C.

Next, the air heater temperature is set (step 3). This is set equal to the substrate temperature immediately before the main heating zone III, and the temperature Tair _{1 of the} air heater 5 is set to a temperature higher than Tbf by a predetermined temperature ΔT (5 to 10 ° C.). For example, Tair ₁ = Tair ₂ = Tbf + 10 ° C. = 170 ° C. Also, the temperature Tair _{3 of the} air heater 10 is Tair ₃ = (melting point of solder +
α) = fixed at 250 ° C.

Then, the first round of the surface heater temperature _{Th 1, Th 2, Th 3} , Th 4
(Reflow conditions) are set (step 5). still,
_{Th 1 = Th 2, Th 3} = Th 4, a C / A = ct. The target substrate temperature Tak of the substrate 20 at the point k (preheating zone I) in FIG.
A conveyor speed CS, the heat capacity of the substrate 20 C / A (= ct) , considered as a function of the temperature Th ₁ surface heater 3. Therefore, the temperature T
Conversely, h ₁ is considered to be a function Th ₁ = f (CS, C / A, Tak) of each of the values CS, C / A, and Tak (1). This function can be determined, for example, by calculating a correlation coefficient by an experiment design method. The simplest expression is expressed by a linear expression relating to Th ₁ as shown in the following expression.

Th ₁ = a ₁ CS + a ₂ C / A + a ₃ Tak + a ₄ (2) The target substrate temperature Tam of the substrate 20 at the point m is the conveyor speed CS, the value C / A (= ct) of the substrate 20, and the surface heater 8. considered as a function of the temperature Th _3, and thus, the temperature Th _3, the respective values CS, C / a, Taf, function of _{Tam Th 3 = f (CS,} C / a, Taf, Tam) ... (3) it is conceivable that. The temperature Th ₃ faces the heater 8 of the heating zone III is also dependent on the temperature Taf of the soaking zone II immediately before such main heating zone III. The simplest formula of the function is similarly represented by a linear equation relating Th ₃ in the formula (2).

Th ₃ = b ₁ CS + b ₂ C / A + b ₃ Taf + b ₄ Tam + b ₅ (4) Accordingly, if the coefficients a _{1 to} a ₄ and b _{1 to} b ₅ are determined, the surface heater 3 in the first preheating zone I The temperature Th ₁ and the temperature Th ₃ of the surface heater 8 in the main heating zone III can be set. These coefficients a _{1 to} a ₄ and b _{1 to} b ₅ are obtained by taking data in advance.

Now, let the coefficients a _{1 to} a ₄ be a ₁ = 126.8, a ₂ = 70.99, a ₃ = 2.
028, a ₄ = 134.27, CS = 0.7m / min, C / A = ct = 1.28,
If Tak = 165 ° C., the temperature Th ₁ of the surface heater 3 becomes Th ₁ = 380 ° C. from the equation (2).

Similarly, the coefficients b _{1 to} b ₅ are calculated as follows: b ₁ = 131.9, b ₂ = 77.5, b ₃ = 1.3
07, b ₄ = 2.584, and _{b 5 = 201.3, CS = 0.7m} / min, C / A = c
Assuming that t = 1.28, Taf = 165 ° C., and Tam = 225 ° C., the temperature Th ₃ of the surface heater 8 becomes Th ₃ = 356 ° C. from the above equation (4).

The temperature control device includes the surface heaters 3 and 4 in the preheating zone I.
Further, the respective temperatures of the surface heaters 8 and 9 in the main heating zone III are set to the set temperatures Th ₁ , Th _2, Th ₃ and Th ₄ , and are raised and adjusted to these temperatures (step 6). The substrate 2 in each of the preheating zone I, the soaking zone II, and the main heating zone III
0, the highest temperature in the first time (n = 1) of the lead 22 is measured by a temperature recording device (reflow checker) 26 (step 7), and based on the measurement result, each temperature Tak = 155 ° C., Taf = 1
Read 60 ° C., Tam = 220 ° C., Tbm = 205 ° C. (step 8).

Next, in step 9, n = n + 1 is set and the process proceeds to the second time, and the conveyor speed CS is corrected (step 10). That is, the conveyor speed CS is corrected and the temperature of the substrate 20 is Tam = 225 ° C.
Then, the temperature is increased by a temperature difference from the measured maximum temperature Tam = 220 ° C. (Tam = 220 + (225−220) = 225 ° C.).

Conveyor speed CS, the temperature Th ₃ of the heater 8, is represented the temperature of the substrate 20 Tam, Taf, function CS = f of the heat capacity _{C / A (Th 3, Tam} , Taf, C / A) as ... (5), CS = (1 / b ₁ ) (Th ₃ −b ₄ Tam−b ₃ Taf + b ₂ C / A−b ₅ ) (6)

In the coefficients b _{1 to} b ₅ of the equation (6), the values b ₁ = 131.9, b _{2
= 77.5, b 3 = 1.307,} b 4 = 2.584, the b ₅ = 201.3, Tam = 22
Substituting 5 ° C., Taf = 160 ° C. and C / A = 0.7 m / min, obtain a conveyor speed CS = 0.65 m / min.

The temperature control device includes the surface heaters 3 and 4 in the preheating zone I.
Then, the respective temperatures of the surface heaters 8 and 9 in the main heating zone III are adjusted to the set temperatures Th ₁ , Th _2, Th ₃ and Th ₄ (step 11). Then, in each of the preheating zone I, the soaking zone II, and the main heating zone III, the second time (n =
Each temperature in 2) is measured by a temperature recording device (reflow checker) 26 (step 12), and for example, a temperature Tbm = 213 ° C. is read from the measurement result (step 13).

Next, in step 14, n = n + 1 = 3 and the third
Proceeding to the second time, the conveyor speed CS is estimated from the results of the first and second measurements by linear approximation so that the lead 22 reaches the target maximum temperature (step 15). That is, this step 15
At the first time (n-2) and the second time (n-
Estimating the conveyor speed CS ³ using a linear approximation represented from the measurement result of 1) the following equation (7).

CS ⁿ = (CS ⁿ⁻¹ −CS ⁿ⁻² ) / (Tbm ⁿ⁻¹ −Tbm ⁿ⁻² ) × (Tbm ⁰ −Tbm ⁿ⁻² ) + CS ⁿ⁻² (7) where the value Tbm ⁰ Indicates a target temperature. The suffix on the right shoulder of each value indicates the number of measurements.

Therefore, the third n = 3 is: CS ³ = (CS ² −CS ¹ ) / (Tbm ² −Tbm ¹ ) × (Tbm ⁰ −Tbm ¹ ) + CS ¹ (8) ^{2 = 0.65 (m / min)} , CS 1 = 0.7
(M / min), Tbm ² = 213 ° C, Tbm ¹ = 205 ° C and Tbm ⁰ = 210 ° C
By substituting the values, obtaining a ^{CS 3 = 0.67 (m / min} ).

The temperature control device includes the surface heaters 3 and 4 in the preheating zone I.
And the temperature Th ₁ and Th surface heater 8 and 9 of the main heating zone III
₃ is adjusted (step 16), and a third measurement is performed by the temperature recording device (reflow checker) 26 (step 1).
7), the number of measurements n is advanced by 1 (step 18). This measurement ends when the target temperature Tbm = 210 ° C. is reached. Then, it is determined whether or not the number of measurements n has exceeded the N-th time (step 19). If the determination result is negative (NO), step
Returning to 12, the conveyor speed CS is set by linear approximation from the third and second measurement results. This further increases accuracy. When the answer to step 19 becomes affirmative (YES) repeatedly for the predetermined number N, the temperature setting ends.

The linear approximation is performed in this manner, and the conveyor speed CS is set so that the lead 22 has the target maximum temperature Tbm ⁰ .
Thus, the target maximum temperature can be set by changing the conveyor speed several times. Further, since the heater temperature is not changed, no temperature adjustment time is required.

(Effects of the Invention) As described above, according to the present invention, when setting reflow conditions for a new substrate in a reflow furnace, the first heater temperature setting of the reflow furnace is determined by using a coefficient obtained by taking data in advance. , Using the conveyor speed, the ratio of the heat capacity of the substrate to the heat transfer area, and the function of the heater temperature represented by the target temperature of the substrate, and setting the second reflow condition using the measurement results. The setting of the reflow conditions after the third time is based on the data of the first conveyor speed and the maximum temperature of the lead of the board and the mounted parts, and the second conveyor speed and the maximum temperature of the lead of the board and the mounted parts. By changing the conveyor speed several times by obtaining linear approximation from the data and setting the conveyor speed so that the leads of the mounted parts reach the target maximum temperature. Thus, the target maximum temperature can be set, and the reflow condition for a new substrate can be easily set, so that even an inexperienced operator can easily set the reflow condition. Furthermore, since the heater temperature is not changed, there is no need for a time for adjusting the temperature, the operation can be easily performed, and there is an excellent effect that the working efficiency can be greatly improved.

[Brief description of the drawings]

FIG. 1 is a block diagram showing one embodiment of a reflow furnace for carrying out a method for setting reflow conditions of a reflow furnace according to the present invention, and FIG. 2 shows heating temperatures in respective heating zones of the reflow furnace of FIG. FIG. 3 is a diagram showing temperature measurement of each part in a state where components are mounted on a board to be soldered by the reflow furnace of FIG. 1, and FIG. 4 is a temperature profile of each part of the mounting board of FIG. Characteristic diagram, FIG. 5 (A)
FIG. 5 (D) is a flowchart showing a procedure for carrying out the method of the present invention, and FIG. 6 is a graph in the case where the conveyor speed is changed to bring the leads of the mounted parts to the desired maximum temperature. 1 ... reflow furnace, 2 ... belt conveyor, 3, 4,
8, 9 ... surface heater, 5, 6, 10 ... air heater, 15 ...
... conveying motor, 20 ... substrate, 21 ... mounted parts, 22 ... lead, 23-25 ... thermocouple, 26 ... temperature recording device (reflow checker).

Claims (1)

(57) [Claims] In setting a reflow condition of a new substrate in a reflow furnace, a first temperature setting of a heater in the reflow furnace is performed by setting a coefficient obtained by taking data in advance, a conveyor speed, a heat capacity of the substrate, and a heat transfer. Using the ratio of the area and the function of the heater temperature represented by the target temperature of the substrate, the measurement result is used to set the second reflow condition, and the third and subsequent reflow conditions are set. Is obtained by linear approximation from the data of the first conveyor speed and the maximum temperature of the lead of the board and the mounted parts, and the second time the data of the conveyor speed and the maximum temperature of the lead of the board and the mounted parts. A method for setting reflow conditions of a reflow furnace, wherein the conveyor speed is set so that a lead reaches a target maximum temperature.